| perlvar - Perl predefined variables |
perlvar - Perl predefined variables
Variable names in Perl can have several formats. Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
:: or '. In this case, the part before the last :: or
' is taken to be a package qualifier; see the perlmod manpage.
A Unicode letter that is not ASCII is not considered to be a letter
unless "use utf8" is in effect, and somewhat more complicated
rules apply; see Identifier parsing in the perldata manpage for details.
Perl variable names may also be a sequence of digits, a single
punctuation character, or the two-character sequence: ^ (caret or
CIRCUMFLEX ACCENT) followed by any one of the characters [][A-Z^_?\].
These names are all reserved for
special uses by Perl; for example, the all-digits names are used
to hold data captured by backreferences after a regular expression
match.
Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
preceded by a caret. These must all be written in the form ${^Foo};
the braces are not optional. ${^Foo} denotes the scalar variable
whose name is considered to be a control-F followed by two o's.
These variables are
reserved for future special uses by Perl, except for the ones that
begin with ^_ (caret-underscore). No
name that begins with ^_ will acquire a special
meaning in any future version of Perl; such names may therefore be
used safely in programs. $^_ itself, however, is reserved.
Perl identifiers that begin with digits or
punctuation characters are exempt from the effects of the package
declaration and are always forced to be in package main; they are
also exempt from strict 'vars' errors. A few other names are also
exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG
In particular, the special ${^_XYZ} variables are always taken
to be in package main, regardless of any package declarations
presently in scope.
The following names have special meaning to Perl. Most punctuation names have reasonable mnemonics, or analogs in the shells. Nevertheless, if you wish to use long variable names, you need only say:
use English;
at the top of your program. This aliases all the short names to the long names in the current package. Some even have medium names, generally borrowed from awk. For more info, please see the English manpage.
Before you continue, note the sort order for variables. In general, we
first list the variables in case-insensitive, almost-lexigraphical
order (ignoring the { or ^ preceding words, as in ${^UNICODE}
or $^T), although $_ and @_ move up to the top of the pile.
For variables with the same identifier, we list it in order of scalar,
array, hash, and bareword.
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)
Here are the places where Perl will assume $_ even if you don't use it:
$_ as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.
All file tests (-f, -d) except for -t, which defaults to STDIN.
See -X in the perlfunc manpage
The pattern matching operations m//, s/// and tr/// (aka y///)
when used without an =~ operator.
The default iterator variable in a foreach loop if no other
variable is supplied.
The implicit iterator variable in the grep() and map() functions.
The implicit variable of given().
The default place to put the next value or input record
when a <FH>, readline, readdir or each
operation's result is tested by itself as the sole criterion of a while
test. Outside a while test, this will not happen.
$_ is a global variable.
However, between perl v5.10.0 and v5.24.0, it could be used lexically by
writing my $_. Making $_ refer to the global $_ in the same scope
was then possible with our $_. This experimental feature was removed and is
now a fatal error, but you may encounter it in older code.
Mnemonic: underline is understood in certain operations.
@_ contains the parameters passed to
that subroutine. Inside a subroutine, @_ is the default array for
the array operators pop and shift.
See the perlsub manpage.
/.../, its elements are
separated by this value. Default is a space. For example, this:
print "The array is: @array\n";
is equivalent to this:
print "The array is: " . join($", @array) . "\n";
Mnemonic: works in double-quoted context.
fork() calls.
Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl would emulate POSIX semantics on Linux systems using LinuxThreads, a partial implementation of POSIX Threads that has since been superseded by the Native POSIX Thread Library (NPTL).
LinuxThreads is now obsolete on Linux, and caching getpid()
like this made embedding perl unnecessarily complex (since you'd have
to manually update the value of $$), so now $$ and getppid()
will always return the same values as the underlying C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until and including the 6.0 release, but after that moved to FreeBSD thread semantics, which are POSIX-like.
To see if your system is affected by this discrepancy check if
getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL returns a false
value. NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells.
On some (but not all) operating systems assigning to $0 modifies
the argument area that the ps program sees. On some platforms you
may have to use special ps options or a different ps to see the
changes. Modifying the $0 is more useful as a way of indicating the
current program state than it is for hiding the program you're
running.
Note that there are platform-specific limitations on the maximum
length of $0. In the most extreme case it may be limited to the
space occupied by the original $0.
In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by ps.
In some platforms this padding may extend all the way to the original
length of the argument area, no matter what you do (this is the case
for example with Linux 2.2).
Note for BSD users: setting $0 does not completely remove ``perl''
from the ps(1) output. For example, setting $0 to "foobar" may
result in "perl: foobar (perl)" (whether both the "perl: " prefix
and the `` (perl)'' suffix are shown depends on your exact BSD variant
and version). This is an operating system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that any
thread may modify its copy of the $0 and the change becomes visible
to ps(1) (assuming the operating system plays along). Note that
the view of $0 the other threads have will not change since they
have their own copies of it.
If the program has been given to perl via the switches -e or -E,
$0 will contain the string "-e".
On Linux as of perl v5.14.0 the legacy process name will be set with
prctl(2), in addition to altering the POSIX name via argv[0] as
perl has done since version 4.000. Now system utilities that read the
legacy process name such as ps, top and killall will recognize the
name you set when assigning to $0. The string you supply will be
cut off at 16 bytes, this is a limitation imposed by Linux.
Mnemonic: same as sh and ksh.
getgid(), and the subsequent ones by getgroups(), one of which may be
the same as the first number.
However, a value assigned to $( must be a single number used to
set the real gid. So the value given by $( should not be assigned
back to $( without being forced numeric, such as by adding zero. Note
that this is different to the effective gid ($)) which does take a
list.
You can change both the real gid and the effective gid at the same
time by using POSIX::setgid(). Changes
to $( require a check to $!
to detect any possible errors after an attempted change.
Mnemonic: parentheses are used to group things. The real gid is the group you left, if you're running setgid.
getegid(), and the subsequent ones by getgroups(),
one of which may be the same as the first number.
Similarly, a value assigned to $) must also be a space-separated
list of numbers. The first number sets the effective gid, and
the rest (if any) are passed to setgroups(). To get the effect of an
empty list for setgroups(), just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty setgroups()
list, say $) = "5 5" .
You can change both the effective gid and the real gid at the same
time by using POSIX::setgid() (use only a single numeric argument).
Changes to $) require a check to $! to detect any possible errors
after an attempted change.
$<, $>, $( and $) can be set only on
machines that support the corresponding set[re][ug]id() routine. $(
and $) can be swapped only on machines supporting setregid().
Mnemonic: parentheses are used to group things. The effective gid is the group that's right for you, if you're running setgid.
POSIX::setuid(). Since
changes to $< require a system call, check $! after a change
attempt to detect any possible errors.
Mnemonic: it's the uid you came from, if you're running setuid.
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uids
You can change both the effective uid and the real uid at the same
time by using POSIX::setuid(). Changes to $> require a check
to $! to detect any possible errors after an attempted change.
$< and $> can be swapped only on machines
supporting setreuid().
Mnemonic: it's the uid you went to, if you're running setuid.
$foo{$x,$y,$z}
it really means
$foo{join($;, $x, $y, $z)}
But don't put
@foo{$x,$y,$z} # a slice--note the @
which means
($foo{$x},$foo{$y},$foo{$z})
Default is ``\034'', the same as SUBSEP in awk. If your keys contain
binary data there might not be any safe value for $;.
Consider using ``real'' multidimensional arrays as described in the perllol manpage.
Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
sort(), see sort in the perlfunc manpage.
Because of this specialness $a and $b don't need to be declared
(using use vars, or our()) even when using the strict 'vars'
pragma. Don't lexicalize them with my $a or my $b if you want to
be able to use them in the sort() comparison block or function.
%ENV contains your current environment. Setting a
value in ENV changes the environment for any child processes
you subsequently fork() off.
As of v5.18.0, both keys and values stored in %ENV are stringified.
my $foo = 1;
$ENV{'bar'} = \$foo;
if( ref $ENV{'bar'} ) {
say "Pre 5.18.0 Behaviour";
} else {
say "Post 5.18.0 Behaviour";
}
Previously, only child processes received stringified values:
my $foo = 1;
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
system($^X, '-e',
q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
This happens because you can't really share arbitrary data structures with foreign processes.
This variable can be used to determine whether the Perl interpreter executing a script is in the right range of versions:
warn "No PerlIO!\n" if $] lt '5.008';
When comparing $], string comparison operators are highly
recommended. The inherent limitations of binary floating point
representation can sometimes lead to incorrect comparisons for some
numbers on some architectures.
See also the documentation of use VERSION and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.
See $^V for a representation of the Perl version as a the version manpage object, which allows more flexible string comparisons.
The main advantage of $] over $^V is that it works the same on any
version of Perl. The disadvantages are that it can't easily be compared
to versions in other formats (e.g. literal v-strings, ``v1.2.3'' or
version objects) and numeric comparisons can occasionally fail; it's good
for string literal version checks and bad for comparing to a variable
that hasn't been sanity-checked.
The $OLD_PERL_VERSION form was added in Perl v5.20.0 for historical
reasons but its use is discouraged. (If your reason to use $] is to
run code on old perls then referring to it as $OLD_PERL_VERSION would
be self-defeating.)
Mnemonic: Is this version of perl in the right bracket?
exec()ed processes, while higher file
descriptors are not. Also, during an
open(), system file descriptors are
preserved even if the open() fails (ordinary file descriptors are
closed before the open() is attempted). The close-on-exec
status of a file descriptor will be decided according to the value of
$^F when the corresponding file, pipe, or socket was opened, not the
time of the exec().
@F contains the fields of each line read in when autosplit
mode is turned on. See the perlrun manpage for the -a switch. This array
is package-specific, and must be declared or given a full package name
if not in package main when running under strict 'vars'.
@INC contains the list of places that the do EXPR,
require, or use constructs look for their library files. It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl.
Prior to Perl 5.26, . -which represents the current directory, was included
in @INC; it has been removed. This change in behavior is documented
in PERL_USE_UNSAFE_INC and it is
not recommended that . be re-added to @INC.
If you need to modify @INC at runtime, you should use the use lib pragma
to get the machine-dependent library properly loaded as well:
use lib '/mypath/libdir/';
use SomeMod;
You can also insert hooks into the file inclusion system by putting Perl
code directly into @INC. Those hooks may be subroutine references,
array references or blessed objects. See require in the perlfunc manpage for details.
%INC contains entries for each filename included via the
do, require, or use operators. The key is the filename
you specified (with module names converted to pathnames), and the
value is the location of the file found. The require
operator uses this hash to determine whether a particular file has
already been included.
If the file was loaded via a hook (e.g. a subroutine reference, see
require in the perlfunc manpage for a description of these hooks), this hook is
by default inserted into %INC in place of a filename. Note, however,
that the hook may have set the %INC entry by itself to provide some more
specific info.
undef to disable
inplace editing.
Mnemonic: value of -i switch.
@ISA which contains a list
of that class's parent classes, if any. This array is simply a list of
scalars, each of which is a string that corresponds to a package name. The
array is examined when Perl does method resolution, which is covered in
the perlobj manpage.
To load packages while adding them to @ISA, see the the parent manpage pragma. The
discouraged the base manpage pragma does this as well, but should not be used except
when compatibility with the discouraged the fields manpage pragma is required.
$^M
as an emergency memory pool after die()ing. Suppose that your Perl
were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
Then
$^M = 'a' x (1 << 16);
would allocate a 64K buffer for use in an emergency. See the INSTALL file in the Perl distribution for information on how to add custom C compilation flags when compiling perl. To discourage casual use of this advanced feature, there is no English long name for this variable.
This variable was added in Perl 5.004.
The value is identical to $Config{'osname'}. See also the Config manpage
and the -V command-line switch documented in the perlrun manpage.
In Windows platforms, $^O is not very helpful: since it is always
MSWin32, it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
Win32::GetOSVersion() (see the Win32 manpage and the perlport manpage) to distinguish
between the variants.
This variable was added in Perl 5.003.
%SIG contains signal handlers for signals. For example:
sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of 'IGNORE' usually has the effect of ignoring the
signal, except for the CHLD signal. See the perlipc manpage for more about
this special case.
Here are some other examples:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
# recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current
# Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
# return??
Be sure not to use a bareword as the name of a signal handler, lest you inadvertently call it.
If your system has the sigaction() function then signal handlers
are installed using it. This means you get reliable signal handling.
The default delivery policy of signals changed in Perl v5.8.0 from immediate (also known as ``unsafe'') to deferred, also known as ``safe signals''. See the perlipc manpage for more information.
Certain internal hooks can be also set using the %SIG hash. The
routine indicated by $SIG{__WARN__} is called when a warning
message is about to be printed. The warning message is passed as the
first argument. The presence of a __WARN__ hook causes the
ordinary printing of warnings to STDERR to be suppressed. You can
use this to save warnings in a variable, or turn warnings into fatal
errors, like this:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;
As the 'IGNORE' hook is not supported by __WARN__, you can
disable warnings using the empty subroutine:
local $SIG{__WARN__} = sub {};
The routine indicated by $SIG{__DIE__} is called when a fatal
exception is about to be thrown. The error message is passed as the
first argument. When a __DIE__ hook routine returns, the exception
processing continues as it would have in the absence of the hook,
unless the hook routine itself exits via a goto &sub, a loop exit,
or a die(). The __DIE__ handler is explicitly disabled during
the call, so that you can die from a __DIE__ handler. Similarly
for __WARN__.
The $SIG{__DIE__} hook is called even inside an eval(). It was
never intended to happen this way, but an implementation glitch made
this possible. This used to be deprecated, as it allowed strange action
at a distance like rewriting a pending exception in $@. Plans to
rectify this have been scrapped, as users found that rewriting a
pending exception is actually a useful feature, and not a bug.
__DIE__/__WARN__ handlers are very special in one respect: they
may be called to report (probable) errors found by the parser. In such
a case the parser may be in inconsistent state, so any attempt to
evaluate Perl code from such a handler will probably result in a
segfault. This means that warnings or errors that result from parsing
Perl should be used with extreme caution, like this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give "
. "backtrace...\n\t"
. "To see backtrace try starting Perl with -MCarp switch";
Here the first line will load Carp unless it is the parser who
called the handler. The second line will print backtrace and die if
Carp was available. The third line will be executed only if Carp was
not available.
Having to even think about the $^S variable in your exception
handlers is simply wrong. $SIG{__DIE__} as currently implemented
invites grievous and difficult to track down errors. Avoid it
and use an END{} or CORE::GLOBAL::die override instead.
See die in the perlfunc manpage, warn in the perlfunc manpage, eval in the perlfunc manpage, and the warnings manpage for additional information.
This variable first appeared in perl v5.6.0; earlier versions of perl
will see an undefined value. Before perl v5.10.0 $^V was represented
as a v-string rather than a the version manpage object.
$^V can be used to determine whether the Perl interpreter executing
a script is in the right range of versions. For example:
warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
While version objects overload stringification, to portably convert
$^V into its string representation, use sprintf()'s "%vd"
conversion, which works for both v-strings or version objects:
printf "version is v%vd\n", $^V; # Perl's version
See the documentation of use VERSION and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.
See also /$] for a decimal representation of the Perl version.
The main advantage of $^V over $] is that, for Perl v5.10.0 or
later, it overloads operators, allowing easy comparison against other
version representations (e.g. decimal, literal v-string, ``v1.2.3'', or
objects). The disadvantage is that prior to v5.10.0, it was only a
literal v-string, which can't be easily printed or compared, whereas
the behavior of $] is unchanged on all versions of Perl.
Mnemonic: use ^V for a version object.
stat() on Windows will
not try to open the file. This means that the link count cannot be
determined and file attributes may be out of date if additional
hardlinks to the file exist. On the other hand, not opening the file
is considerably faster, especially for files on network drives.
This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use ``sloppy'' stat() by
default. See the documentation for -f in
perlrun for more information about site
customization.
This variable was added in Perl v5.10.0.
argv[0] or (where supported) /proc/self/exe.
Depending on the host operating system, the value of $^X may be
a relative or absolute pathname of the perl program file, or may
be the string used to invoke perl but not the pathname of the
perl program file. Also, most operating systems permit invoking
programs that are not in the PATH environment variable, so there
is no guarantee that the value of $^X is in PATH. For VMS, the
value may or may not include a version number.
You usually can use the value of $^X to re-invoke an independent
copy of the same perl that is currently running, e.g.,
@first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or capturing of the output of commands, so this complex statement may not be portable.
It is not safe to use the value of $^X as a path name of a file,
as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command. To convert the value of $^X to a path name, use the
following statements:
# Build up a set of file names (not command names).
use Config;
my $this_perl = $^X;
if ($^O ne 'VMS') {
$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;
}
Because many operating systems permit anyone with read access to
the Perl program file to make a copy of it, patch the copy, and
then execute the copy, the security-conscious Perl programmer
should take care to invoke the installed copy of perl, not the
copy referenced by $^X. The following statements accomplish
this goal, and produce a pathname that can be invoked as a
command or referenced as a file.
use Config;
my $secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS') {
$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;
}
Most of the special variables related to regular expressions are side effects. Perl sets these variables when it has a successful match, so you should check the match result before using them. For instance:
if( /P(A)TT(ER)N/ ) {
print "I found $1 and $2\n";
}
These variables are read-only and dynamically-scoped, unless we note otherwise.
The dynamic nature of the regular expression variables means that their value is limited to the block that they are in, as demonstrated by this bit of code:
my $outer = 'Wallace and Grommit';
my $inner = 'Mutt and Jeff';
my $pattern = qr/(\S+) and (\S+)/;
sub show_n { print "\$1 is $1; \$2 is $2\n" }
{
OUTER:
show_n() if $outer =~ m/$pattern/;
INNER: {
show_n() if $inner =~ m/$pattern/;
}
show_n();
}
The output shows that while in the OUTER block, the values of $1
and $2 are from the match against $outer. Inside the INNER
block, the values of $1 and $2 are from the match against
$inner, but only until the end of the block (i.e. the dynamic
scope). After the INNER block completes, the values of $1 and
$2 return to the values for the match against $outer even though
we have not made another match:
$1 is Wallace; $2 is Grommit
$1 is Mutt; $2 is Jeff
$1 is Wallace; $2 is Grommit
Traditionally in Perl, any use of any of the three variables $`, $&
or $' (or their use English equivalents) anywhere in the code, caused
all subsequent successful pattern matches to make a copy of the matched
string, in case the code might subsequently access one of those variables.
This imposed a considerable performance penalty across the whole program,
so generally the use of these variables has been discouraged.
In Perl 5.6.0 the @- and @+ dynamic arrays were introduced that
supply the indices of successful matches. So you could for example do
this:
$str =~ /pattern/;
print $`, $&, $'; # bad: performance hit
print # good: no performance hit
substr($str, 0, $-[0]),
substr($str, $-[0], $+[0]-$-[0]),
substr($str, $+[0]);
In Perl 5.10.0 the /p match operator flag and the ${^PREMATCH},
${^MATCH}, and ${^POSTMATCH} variables were introduced, that allowed
you to suffer the penalties only on patterns marked with /p.
In Perl 5.18.0 onwards, perl started noting the presence of each of the three variables separately, and only copied that part of the string required; so in
$`; $&; "abcdefgh" =~ /d/
perl would only copy the ``abcd'' part of the string. That could make a big difference in something like
$str = 'x' x 1_000_000;
$&; # whoops
$str =~ /x/g # one char copied a million times, not a million chars
In Perl 5.20.0 a new copy-on-write system was enabled by default, which finally fixes all performance issues with these three variables, and makes them safe to use anywhere.
The Devel::NYTProf and Devel::FindAmpersand modules can help you
find uses of these problematic match variables in your code.
Note there is a distinction between a capture buffer which matches
the empty string a capture buffer which is optional. Eg, (x?) and
(x)? The latter may be undef, the former not.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1 index is equivalent to $2, etc.
if ("foal"=~/(.)(.)(.)(.)/) {
print join "-", @{^CAPTURE};
}
should output ``f-o-a-l''.
See also $<digits ($1, $2, ...)>, %{^CAPTURE} and %{^CAPTURE_ALL}.
Note that unlike most other regex magic variables there is no single
letter equivalent to @{^CAPTURE}.
This variable was added in 5.25.7
eval() enclosed by the current
BLOCK).
See Performance issues above for the serious performance implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: like & in some editors.
$& ($MATCH) except that it does not incur the
performance penalty associated with that variable.
See Performance issues above.
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier. In Perl v5.20, the /p modifier does nothing, so
${^MATCH} does the same thing as $MATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
eval
enclosed by the current BLOCK.
See Performance issues above for the serious performance implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: ` often precedes a quoted string.
$` ($PREMATCH) except that it does not incur the
performance penalty associated with that variable.
See Performance issues above.
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier. In Perl v5.20, the /p modifier does nothing, so
${^PREMATCH} does the same thing as $PREMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
eval()
enclosed by the current BLOCK). Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
See Performance issues above for the serious performance implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: ' often follows a quoted string.
$' ($POSTMATCH) except that it does not incur the
performance penalty associated with that variable.
See Performance issues above.
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier. In Perl v5.20, the /p modifier does nothing, so
${^POSTMATCH} does the same thing as $POSTMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$1, $2, ...) which has a defined value.
This is useful if you don't know which one of a set of alternative patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.
$+. For example in
"ab" =~ /^((.)(.))$/
we have
$1,$^N have the value "ab"
$2 has the value "a"
$3,$+ have the value "b"
This is primarily used inside (?{...}) blocks for examining text
recently matched. For example, to effectively capture text to a variable
(in addition to $1, $2, etc.), replace (...) with
(?:(...)(?{ $var = $^N }))
By setting and then using $var in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.
This variable was added in Perl v5.8.0.
Mnemonic: the (possibly) Nested parenthesis that most recently closed.
$+[0] is
the offset into the string of the end of the entire match. This
is the same value as what the pos function returns when called
on the variable that was matched against. The nth element
of this array holds the offset of the nth submatch, so
$+[1] is the offset past where $1 ends, $+[2] the offset
past where $2 ends, and so on. You can use $#+ to determine
how many subgroups were in the last successful match. See the
examples given for the @- variable.
This variable was added in Perl v5.6.0.
@+, the %+ hash allows access to the named capture
buffers, should they exist, in the last successful match in the
currently active dynamic scope.
For example, $+{foo} is equivalent to $1 after the following match:
'foo' =~ /(?<foo>foo)/;
The keys of the %+ hash list only the names of buffers that have
captured (and that are thus associated to defined values).
If multiple distinct capture groups have the same name, then
$+{NAME} will refer to the leftmost defined group in the match.
The underlying behaviour of %+ is provided by the
the Tie::Hash::NamedCapture manpage module.
Note: %- and %+ are tied views into a common internal hash
associated with the last successful regular expression. Therefore mixing
iterative access to them via each may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
This variable was added in Perl v5.10.0. The %{^CAPTURE} alias was
added in 5.25.7.
This variable is read-only and dynamically-scoped.
$-[0] is the offset of the start of the last successful match.
$-[n] is the offset of the start of the substring matched by
n-th subpattern, or undef if the subpattern did not match.
Thus, after a match against $_, $& coincides with substr $_, $-[0],
$+[0] - $-[0]. Similarly, $n coincides with substr $_, $-[n],
$+[n] - $-[n] if $-[n] is defined, and $+ coincides with
substr $_, $-[$#-], $+[$#-] - $-[$#-]. One can use $#- to find the
last matched subgroup in the last successful match. Contrast with
$#+, the number of subgroups in the regular expression. Compare
with @+.
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
$-[0] is the offset into the string of the beginning of the
entire match. The nth element of this array holds the offset
of the nth submatch, so $-[1] is the offset where $1
begins, $-[2] the offset where $2 begins, and so on.
After a match against some variable $var:
$` is the same as substr($var, 0, $-[0])$& is the same as substr($var, $-[0], $+[0] - $-[0])$' is the same as substr($var, $+[0])$1 is the same as substr($var, $-[1], $+[1] - $-[1])$2 is the same as substr($var, $-[2], $+[2] - $-[2])$3 is the same as substr($var, $-[3], $+[3] - $-[3])This variable was added in Perl v5.6.0.
%+, this variable allows access to the named capture groups
in the last successful match in the currently active dynamic scope. To
each capture group name found in the regular expression, it associates a
reference to an array containing the list of values captured by all
buffers with that name (should there be several of them), in the order
where they appear.
Here's an example:
if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
foreach my $bufname (sort keys %-) {
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
(defined($ary->[$idx])
? "'$ary->[$idx]'"
: "undef"),
"\n";
}
}
}
would print out:
$-{A}[0] : '1'
$-{A}[1] : '3'
$-{B}[0] : '2'
$-{B}[1] : '4'
The keys of the %- hash correspond to all buffer names found in
the regular expression.
The behaviour of %- is implemented via the
the Tie::Hash::NamedCapture manpage module.
Note: %- and %+ are tied views into a common internal hash
associated with the last successful regular expression. Therefore mixing
iterative access to them via each may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
This variable was added in Perl v5.10.0. The %{^CAPTURE_ALL} alias was
added in 5.25.7.
This variable is read-only and dynamically-scoped.
(?{ code })
regular expression assertion (see the perlre manpage). May be written to.
This variable was added in Perl 5.005.
This variable was added in Perl v5.30.0.
re 'debug' module is loaded. See the re manpage for details.
This variable was added in Perl v5.10.0.
This variable was added in Perl v5.10.0.
Variables that depend on the currently selected filehandle may be set
by calling an appropriate object method on the IO::Handle object,
although this is less efficient than using the regular built-in
variables. (Summary lines below for this contain the word HANDLE.)
First you must say
use IO::Handle;
after which you may use either
method HANDLE EXPR
or more safely,
HANDLE->method(EXPR)
Each method returns the old value of the IO::Handle attribute. The
methods each take an optional EXPR, which, if supplied, specifies the
new value for the IO::Handle attribute in question. If not
supplied, most methods do nothing to the current value--except for
autoflush(), which will assume a 1 for you, just to be different.
Because loading in the IO::Handle class is an expensive operation,
you should learn how to use the regular built-in variables.
A few of these variables are considered ``read-only''. This means that if you try to assign to this variable, either directly or indirectly through a reference, you'll raise a run-time exception.
You should be very careful when modifying the default values of most special variables described in this document. In most cases you want to localize these variables before changing them, since if you don't, the change may affect other modules which rely on the default values of the special variables that you have changed. This is one of the correct ways to read the whole file at once:
open my $fh, "<", "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;
But the following code is quite bad:
open my $fh, "<", "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;
since some other module, may want to read data from some file in the
default ``line mode'', so if the code we have just presented has been
executed, the global value of $/ is now changed for any other code
running inside the same Perl interpreter.
Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short {} block, you should create one yourself. For
example:
my $content = '';
open my $fh, "<", "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;
Here is an example of how your own code can go broken:
for ( 1..3 ){
$\ = "\r\n";
nasty_break();
print "$_";
}
sub nasty_break {
$\ = "\f";
# do something with $_
}
You probably expect this code to print the equivalent of
"1\r\n2\r\n3\r\n"
but instead you get:
"1\f2\f3\f"
Why? Because nasty_break() modifies $\ without localizing it
first. The value you set in nasty_break() is still there when you
return. The fix is to add local() so the value doesn't leak out of
nasty_break():
local $\ = "\f";
It's easy to notice the problem in such a short example, but in more complicated code you are looking for trouble if you don't localize changes to the special variables.
<>.
@ARGV contains the command-line arguments intended for
the script. $#ARGV is generally the number of arguments minus
one, because $ARGV[0] is the first argument, not the program's
command name itself. See $0 for the command name.
@ARGV. Usually written as the null filehandle in the angle operator
<>. Note that currently ARGV only has its magical effect
within the <> operator; elsewhere it is just a plain filehandle
corresponding to the last file opened by <>. In particular,
passing \*ARGV as a parameter to a function that expects a filehandle
may not cause your function to automatically read the contents of all the
files in @ARGV.
$_. See
the perlrun manpage for the -i switch.
undef.
You cannot call output_field_separator() on a handle, only as a
static method. See IO::Handle.
Mnemonic: what is printed when there is a ``,'' in your print statement.
Each filehandle in Perl counts the number of lines that have been read
from it. (Depending on the value of $/, Perl's idea of what
constitutes a line may not match yours.) When a line is read from a
filehandle (via readline() or <>), or when tell() or
seek() is called on it, $. becomes an alias to the line counter
for that filehandle.
You can adjust the counter by assigning to $., but this will not
actually move the seek pointer. Localizing $. will not localize
the filehandle's line count. Instead, it will localize perl's notion
of which filehandle $. is currently aliased to.
$. is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening close(). For more
details, see I/O Operators in the perlop manpage. Because <> never does
an explicit close, line numbers increase across ARGV files (but see
examples in eof in the perlfunc manpage).
You can also use HANDLE->input_line_number(EXPR) to access the
line counter for a given filehandle without having to worry about
which handle you last accessed.
Mnemonic: many programs use ``.'' to mean the current line number.
undef to read through the end of file. Setting it to "\n\n"
means something slightly different than setting to "", if the file
contains consecutive empty lines. Setting to "" will treat two or
more consecutive empty lines as a single empty line. Setting to
"\n\n" will blindly assume that the next input character belongs to
the next paragraph, even if it's a newline.
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regex. awk has to
be better for something. :-)
Setting $/ to an empty string -- the so-called paragraph mode -- merits
special attention. When $/ is set to "" and the entire file is read in
with that setting, any sequence of consecutive newlines "\n\n" at the
beginning of the file is discarded. With the exception of the final record in
the file, each sequence of characters ending in two or more newlines is
treated as one record and is read in to end in exactly two newlines. If the
last record in the file ends in zero or one consecutive newlines, that record
is read in with that number of newlines. If the last record ends in two or
more consecutive newlines, it is read in with two newlines like all preceding
records.
Suppose we wrote the following string to a file:
my $string = "\n\n\n";
$string .= "alpha beta\ngamma delta\n\n\n";
$string .= "epsilon zeta eta\n\n";
$string .= "theta\n";
my $file = 'simple_file.txt';
open my $OUT, '>', $file or die;
print $OUT $string;
close $OUT or die;
Now we read that file in paragraph mode:
local $/ = ""; # paragraph mode
open my $IN, '<', $file or die;
my @records = <$IN>;
close $IN or die;
@records will consist of these 3 strings:
(
"alpha beta\ngamma delta\n\n",
"epsilon zeta eta\n\n",
"theta\n",
)
Setting $/ to a reference to an integer, scalar containing an
integer, or scalar that's convertible to an integer will attempt to
read records instead of lines, with the maximum record size being the
referenced integer number of characters. So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;
will read a record of no more than 32768 characters from $fh. If you're not reading from a record-oriented file (or your OS doesn't have record-oriented files), then you'll likely get a full chunk of data with every read. If a record is larger than the record size you've set, you'll get the record back in pieces. Trying to set the record size to zero or less is deprecated and will cause $/ to have the value of ``undef'', which will cause reading in the (rest of the) whole file.
As of 5.19.9 setting $/ to any other form of reference will throw a
fatal exception. This is in preparation for supporting new ways to set
$/ in the future.
On VMS only, record reads bypass PerlIO layers and any associated buffering, so you must not mix record and non-record reads on the same filehandle. Record mode mixes with line mode only when the same buffering layer is in use for both modes.
You cannot call input_record_separator() on a handle, only as a
static method. See IO::Handle.
See also Newlines in the perlport manpage. Also see $..
Mnemonic: / delimits line boundaries when quoting poetry.
undef.
You cannot call output_record_separator() on a handle, only as a
static method. See IO::Handle.
Mnemonic: you set $\ instead of adding ``\n'' at the end of the print.
Also, it's just like $/, but it's what you get ``back'' from Perl.
$| tells you only whether you've asked Perl explicitly to
flush after each write). STDOUT will typically be line buffered if
output is to the terminal and block buffered otherwise. Setting this
variable is useful primarily when you are outputting to a pipe or
socket, such as when you are running a Perl program under rsh and
want to see the output as it's happening. This has no effect on input
buffering. See getc in the perlfunc manpage for that. See select in the perlfunc manpage on
how to select the output channel. See also the IO::Handle manpage.
Mnemonic: when you want your pipes to be piping hot.
<HANDLE>, readline, tell, eof and seek.
This is the same handle that $. and tell and eof without arguments
use. It is also the handle used when Perl appends ``, <STDIN> line 1'' to
an error or warning message.
This variable was added in Perl v5.18.0.
The special variables for formats are a subset of those for filehandles. See the perlform manpage for more information about Perl's formats.
write() accumulator for format() lines.
A format contains formline() calls that put their result into
$^A. After calling its format, write() prints out the contents
of $^A and empties. So you never really see the contents of $^A
unless you call formline() yourself and then look at it. See
the perlform manpage and formline PICTURE,LIST in the perlfunc manpage.
format_formfeed(EXPR)\f.
You cannot call format_formfeed() on a handle, only as a static
method. See IO::Handle.
format_page_number(EXPR)Mnemonic: % is page number in nroff.
format_lines_left(EXPR)Mnemonic: lines_on_page - lines_printed.
^) in a format. The default is
`` \n-'', to break on a space, newline, or a hyphen.
You cannot call format_line_break_characters() on a handle, only as
a static method. See IO::Handle.
Mnemonic: a ``colon'' in poetry is a part of a line.
format_lines_per_page(EXPR)Mnemonic: = has horizontal lines.
format_top_name(EXPR)_TOP
appended. For example, the default format top name for the STDOUT
filehandle is STDOUT_TOP.
Mnemonic: points to top of page.
format_name(EXPR)STDOUT
filehandle is just STDOUT.
Mnemonic: brother to $^.
The variables $@, $!, $^E, and $? contain information
about different types of error conditions that may appear during
execution of a Perl program. The variables are shown ordered by
the ``distance'' between the subsystem which reported the error and
the Perl process. They correspond to errors detected by the Perl
interpreter, C library, operating system, or an external program,
respectively.
To illustrate the differences between these variables, consider the following Perl expression, which uses a single-quoted string. After execution of this statement, perl may have set all four special error variables:
eval q{
open my $pipe, "/cdrom/install |" or die $!;
my @res = <$pipe>;
close $pipe or die "bad pipe: $?, $!";
};
When perl executes the eval() expression, it translates the
open(), <PIPE>, and close calls in the C run-time library
and thence to the operating system kernel. perl sets $! to
the C library's errno if one of these calls fails.
$@ is set if the string to be eval-ed did not compile (this may
happen if open or close were imported with bad prototypes), or
if Perl code executed during evaluation die()d. In these cases the
value of $@ is the compile error, or the argument to die (which
will interpolate $! and $?). (See also the Fatal manpage, though.)
Under a few operating systems, $^E may contain a more verbose error
indicator, such as in this case, ``CDROM tray not closed.'' Systems that
do not support extended error messages leave $^E the same as $!.
Finally, $? may be set to a non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific error
conditions encountered by the program (the program's exit() value).
The lower eight bits reflect mode of failure, like signal death and
core dump information. See wait(2) for details. In contrast to
$! and $^E, which are set only if an error condition is detected,
the variable $? is set on each wait or pipe close,
overwriting the old value. This is more like $@, which on every
eval() is always set on failure and cleared on success.
For more details, see the individual descriptions at $@, $!,
$^E, and $?.
``)
command, successful call to wait() or waitpid(), or from the
system() operator. On POSIX-like systems this value can be decoded
with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
WSTOPSIG and WIFCONTINUED functions provided by the the POSIX manpage module.
Under VMS this reflects the actual VMS exit status; i.e. it is the
same as $? when the pragma use vmsish 'status' is in effect.
This variable was added in Perl v5.10.0.
/$! under only VMS, OS/2, and Win32 (and
for MacPerl). On all other platforms, $^E is always just the same
as $!.
Under VMS, $^E provides the VMS status value from the last system
error. This is more specific information about the last system error
than that provided by $!. This is particularly important when $!
is set to EVMSERR.
Under OS/2, $^E is set to the error code of the last call to OS/2
API either via CRT, or directly from perl.
Under Win32, $^E always returns the last error information reported
by the Win32 call GetLastError() which describes the last error
from within the Win32 API. Most Win32-specific code will report errors
via $^E. ANSI C and Unix-like calls set errno and so most
portable Perl code will report errors via $!.
Caveats mentioned in the description of /$! generally apply to
$^E, also.
This variable was added in Perl 5.003.
Mnemonic: Extra error explanation.
$^S State
--------- -------------------------------------
undef Parsing module, eval, or main program
true (1) Executing an eval
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers.
The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
the undef value does not indicate whether exceptions are being caught,
since compilation of the main program does not catch exceptions.
This variable was added in Perl 5.004.
See also the warnings manpage.
Mnemonic: related to the -w switch.
use warnings pragma.
It has the same scoping as the $^H and %^H variables. The exact
values are considered internal to the the warnings manpage pragma and may change
between versions of Perl.
This variable was added in Perl v5.6.0.
$! retrieves the current value
of the C errno integer variable.
If $! is assigned a numerical value, that value is stored in errno.
When referenced as a string, $! yields the system error string
corresponding to errno.
Many system or library calls set errno if they fail,
to indicate the cause of failure. They usually do not
set errno to zero if they succeed. This means errno,
hence $!, is meaningful only immediately after a failure:
if (open my $fh, "<", $filename) {
# Here $! is meaningless.
...
}
else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# $! is meaningless.
Here, meaningless means that $! may be unrelated to the outcome
of the open() operator. Assignment to $! is similarly ephemeral.
It can be used immediately before invoking the die() operator,
to set the exit value, or to inspect the system error string
corresponding to error n, or to restore $! to a meaningful state.
Mnemonic: What just went bang?
%! has a true value only if $! is set to that
value. For example, $!{ENOENT} is true if and only if the current
value of $! is ENOENT; that is, if the most recent error was ``No
such file or directory'' (or its moral equivalent: not all operating
systems give that exact error, and certainly not all languages). The
specific true value is not guaranteed, but in the past has generally
been the numeric value of $!. To check if a particular key is
meaningful on your system, use exists $!{the_key}; for a list of legal
keys, use keys %!. See the Errno manpage for more information, and also see
$!.
This variable was added in Perl 5.005.
``) command,
successful call to wait() or waitpid(), or from the system()
operator. This is just the 16-bit status word returned by the
traditional Unix wait() system call (or else is made up to look
like it). Thus, the exit value of the subprocess is really ($? >>
8), and $? & 127 gives which signal, if any, the process died
from, and $? & 128 reports whether there was a core dump.
Additionally, if the h_errno variable is supported in C, its value
is returned via $? if any gethost*() function fails.
If you have installed a signal handler for SIGCHLD, the
value of $? will usually be wrong outside that handler.
Inside an END subroutine $? contains the value that is going to be
given to exit(). You can modify $? in an END subroutine to
change the exit status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma use vmsish 'status' makes $? reflect the
actual VMS exit status, instead of the default emulation of POSIX
status; see $? in the perlvms manpage for details.
Mnemonic: similar to sh and ksh.
eval operator, i.e. the last exception that
was caught. For eval BLOCK, this is either a runtime error message or the
string or reference die was called with. The eval STRING form also
catches syntax errors and other compile time exceptions.
If no error occurs, eval sets $@ to the empty string.
Warning messages are not collected in this variable. You can, however,
set up a routine to process warnings by setting $SIG{__WARN__} as
described in %SIG.
Mnemonic: Where was the error ``at''?
These variables provide information about the current interpreter state.
AUTOLOAD at compile
time rather than normal, deferred loading. Setting
$^C = 1 is similar to calling B::minus_c.
This variable was added in Perl v5.6.0.
$^D = 10 or $^D = "st". See
-Dnumber in the perlrun manpage. The contents of this variable also affects the
debugger operation. See Debugger Internals in the perldebguts manpage.
Mnemonic: value of -D switch.
It used to hold the object reference to the Encode object that was
used to convert the source code to Unicode.
Its purpose was to allow your non-ASCII Perl scripts not to have to be written in UTF-8; this was useful before editors that worked on UTF-8 encoded text were common, but that was long ago. It caused problems, such as affecting the operation of other modules that weren't expecting it, causing general mayhem.
If you need something like this functionality, it is recommended that use you a simple source filter, such as the Filter::Encoding manpage.
If you are coming here because code of yours is being adversely affected by someone's use of this variable, you can usually work around it by doing this:
local ${^ENCODING};
near the beginning of the functions that are getting broken. This undefines the variable during the scope of execution of the including function.
This variable was added in Perl 5.8.2 and removed in 5.26.0.
Setting it to anything other than undef was made fatal in Perl 5.28.0.
Possible values are:
PerlInterpreter* is being constructed via perl_construct. This
value is mostly there for completeness and for use via the
underlying C variable PL_phase. It's not really possible for Perl
code to be executed unless construction of the interpreter is
finished.
BEGIN block executed directly or indirectly from during the
compile-time of the top-level program.
This phase is not called ``BEGIN'' to avoid confusion with
BEGIN-blocks, as those are executed during compile-time of any
compilation unit, not just the top-level program. A new, localised
compile-time entered at run-time, for example by constructs as
eval "use SomeModule" are not global interpreter phases, and
therefore aren't reflected by ${^GLOBAL_PHASE}.
CHECK blocks.
INIT-blocks, not CHECK blocks.
PL_main_root.
END blocks.
Also note that there's no value for UNITCHECK-blocks. That's because those are run for each compilation unit individually, and therefore is not a global interpreter phase.
Not every program has to go through each of the possible phases, but transition from one phase to another can only happen in the order described in the above list.
An example of all of the phases Perl code can see:
BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
{
package Print::Phase;
sub new {
my ($class, $time) = @_;
return bless \$time, $class;
}
sub DESTROY {
my $self = shift;
print "$$self: ${^GLOBAL_PHASE}\n";
}
}
print "run-time: ${^GLOBAL_PHASE}\n";
my $runtime = Print::Phase->new(
"lexical variables are garbage collected before END"
);
END { print "end-time: ${^GLOBAL_PHASE}\n" }
our $destruct = Print::Phase->new(
"package variables are garbage collected after END"
);
This will print out
compile-time: START
check-time: CHECK
init-time: INIT
run-time: RUN
lexical variables are garbage collected before END: RUN
end-time: END
package variables are garbage collected after END: DESTRUCT
This variable was added in Perl 5.14.0.
This variable contains compile-time hints for the Perl interpreter. At the end of compilation of a BLOCK the value of this variable is restored to the value when the interpreter started to compile the BLOCK.
When perl begins to parse any block construct that provides a lexical scope
(e.g., eval body, required file, subroutine body, loop body, or conditional
block), the existing value of $^H is saved, but its value is left unchanged.
When the compilation of the block is completed, it regains the saved value.
Between the points where its value is saved and restored, code that
executes within BEGIN blocks is free to change the value of $^H.
This behavior provides the semantic of lexical scoping, and is used in,
for instance, the use strict pragma.
The contents should be an integer; different bits of it are used for different pragmatic flags. Here's an example:
sub add_100 { $^H |= 0x100 }
sub foo {
BEGIN { add_100() }
bar->baz($boon);
}
Consider what happens during execution of the BEGIN block. At this point
the BEGIN block has already been compiled, but the body of foo() is still
being compiled. The new value of $^H
will therefore be visible only while
the body of foo() is being compiled.
Substitution of BEGIN { add_100() } block with:
BEGIN { require strict; strict->import('vars') }
demonstrates how use strict 'vars' is implemented. Here's a conditional
version of the same lexical pragma:
BEGIN {
require strict; strict->import('vars') if $condition
}
This variable was added in Perl 5.003.
%^H hash provides the same scoping semantic as $^H. This makes
it useful for implementation of lexically scoped pragmas. See
the perlpragma manpage. All the entries are stringified when accessed at
runtime, so only simple values can be accommodated. This means no
pointers to objects, for example.
When putting items into %^H, in order to avoid conflicting with other
users of the hash there is a convention regarding which keys to use.
A module should use only keys that begin with the module's name (the
name of its main package) and a ``/'' character. For example, a module
Foo::Bar should use keys such as Foo::Bar/baz.
This variable was added in Perl v5.6.0.
\0 byte, the first part describes the input layers, the second
part describes the output layers.
This variable was added in Perl v5.8.0.
DB::DB() subroutine to be called for
each statement executed. Also causes saving source code lines (like
0x400).
goto &subroutine as well.
@{"_<$filename"}.
Some bits may be relevant at compile-time only, some at run-time only. This is a new mechanism and the details may change. See also the perldebguts manpage.
This variable is read-only.
This variable was added in Perl v5.8.0.
This variable is read-only.
This variable was added in Perl v5.28.0.
-C switch for more information about
the possible values.
This variable is set during Perl startup and is thereafter read-only.
This variable was added in Perl v5.8.2.
This variable was added in Perl v5.8.9. It is subject to change or removal without notice, but is currently used to avoid recalculating the boundaries of multi-byte UTF-8-encoded characters.
-CL command-line
switch); see the perlrun manpage for more info on this.
This variable was added in Perl v5.8.8.
Deprecating a variable announces the intent of the perl maintainers to eventually remove the variable from the language. It may still be available despite its status. Using a deprecated variable triggers a warning.
Once a variable is removed, its use triggers an error telling you the variable is unsupported.
See the perldiag manpage for details about error messages.
$# was a variable that could be used to format printed numbers.
After a deprecation cycle, its magic was removed in Perl v5.10.0 and
using it now triggers a warning: $# is no longer supported.
This is not the sigil you use in front of an array name to get the
last index, like $#array. That's still how you get the last index
of an array in Perl. The two have nothing to do with each other.
Deprecated in Perl 5.
Removed in Perl v5.10.0.
$* was a variable that you could use to enable multiline matching.
After a deprecation cycle, its magic was removed in Perl v5.10.0.
Using it now triggers a warning: $* is no longer supported.
You should use the /s and /m regexp modifiers instead.
Deprecated in Perl 5.
Removed in Perl v5.10.0.
index() and substr() functions.
As of release 5 of Perl, assignment to $[ is treated as a compiler
directive, and cannot influence the behavior of any other file.
(That's why you can only assign compile-time constants to it.)
Its use is highly discouraged.
Prior to Perl v5.10.0, assignment to $[ could be seen from outer lexical
scopes in the same file, unlike other compile-time directives (such as
the strict manpage). Using local() on it would bind its value strictly to a lexical
block. Now it is always lexically scoped.
As of Perl v5.16.0, it is implemented by the arybase module.
As of Perl v5.30.0, or under use v5.16, or no feature "array_base",
$[ no longer has any effect, and always contains 0.
Assigning 0 to it is permitted, but any other value will produce an error.
Mnemonic: [ begins subscripts.
Deprecated in Perl v5.12.0.
| perlvar - Perl predefined variables |