PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
NAME
perlvar - Perl predefined variables
DESCRIPTION
The Syntax of Variable Names
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 perlmod.
Perl variable names may also be a sequence of digits or a single punctuation or control character. 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. Perl has a special syntax for the single-
control-character names: It understands "^X" (caret "X") to mean the control-"X" character. For example, the
notation $^W (dollar-sign caret "W") is the scalar variable whose name is the single character control-"W".
This is better than typing a literal control-"W" into your program.
Since Perl 5.6, Perl variable names may be alphanumeric strings that begin with control characters (or better
yet, a caret). These variables must be written in the form "${^Foo}"; the braces are not optional. "${^Foo}"
denotes the scalar variable whose name is 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 "^_" (control-underscore or caret-
underscore). No control-character name that begins with "^_" will acquire a special meaning in any future
version of Perl; such names may therefore be used safely in programs. $^_ itself, however, is reserved.
Perl identifiers that begin with digits, control characters, or punctuation characters are exempt from the
effects of the "package" declaration and are always forced to be in package "main"; they are also exempt from
"strict 'vars'" errors. A few other names are also exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG
In particular, the special "${^_XYZ}" variables are always taken to be in package "main", regardless of any
"package" declarations presently in scope.
SPECIAL VARIABLES
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. To avoid a performance hit, if you don't need the
$PREMATCH, $MATCH, or $POSTMATCH it's best to use the "English" module without them:
use English '-no_match_vars';
Before you continue, note the sort order for variables. In general, we first list the variables in case-
insensitive, almost-lexigraphical order (ignoring the "{" or "^" preceding words, as in "${^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.
General Variables
$ARG
chomp($_)
Here are the places where Perl will assume $_ even if you don't use it:
· The following functions use $_ as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, glob, hex, int, lc,
lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, quotemeta, readlink, readpipe, ref,
require, reverse (in scalar context only), rmdir, sin, split (on 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 perlfunc
· 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 an input record when a "<FH>" operation's result is tested by itself as
the sole criterion of a "while" test. Outside a "while" test, this will not happen.
As $_ is a global variable, this may lead in some cases to unwanted side-effects. As of perl 5.10,
you can now use a lexical version of $_ by declaring it in a file or in a block with "my". Moreover,
declaring "our $_" restores the global $_ in the current scope.
Mnemonic: underline is understood in certain operations.
@ARG
@_ Within a subroutine the array @_ contains the parameters passed to that subroutine. Inside a
subroutine, @_ is the default array for the array operators "push", "pop", "shift", and "unshift".
See perlsub.
$LIST_SEPARATOR
$" When an array or an array slice is interpolated into a double-quoted string or a similar context such
as "/.../", its elements are separated by this value. Default is a space. For example, this:
print "The array is: @array\n";
is equivalent to this:
print "The array is: " . join($", @array) . "\n";
Mnemonic: works in double-quoted context.
$PROCESS_ID
$PID
$$ The process number of the Perl running this script. Though you can set this variable, doing so is
generally discouraged, although it can be invaluable for some testing purposes. It will be reset
automatically across "fork()" calls.
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.
$PROGRAM_NAME
$0 Contains the name of the program being executed.
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 5.14 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.
$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a machine that supports membership in multiple groups
simultaneously, gives a space separated list of groups you are in. The first number is the one
returned by "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.
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.
$REAL_USER_ID
$UID
$< The real uid of this process. You can change both the real uid and the effective uid at the same time
by using "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.
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. For example:
$< = $>; # 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.
$SUBSCRIPT_SEPARATOR
$SUBSEP
$; The subscript separator for multidimensional array emulation. If you refer to a hash element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. If your keys contain binary data there might not be any
safe value for $;.
$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2. System file descriptors are passed to "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 The array @F contains the fields of each line read in when autosplit mode is turned on. See perlrun
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 The array @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, followed by ".", to represent the current
directory. ("." will not be appended if taint checks are enabled, either by "-T" or by "-t".) If you
need to modify this at runtime, you should use the "use lib" pragma to get the machine-dependent
library properly loaded also:
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
perlfunc for details.
%INC The hash %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 perlfunc 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.
$INPLACE_EDIT
$^I The current value of the inplace-edit extension. Use "undef" to disable inplace editing.
Mnemonic: value of -i switch.
$^M By default, running out of memory is an untrappable, fatal error. However, if suitably built, Perl
can use the contents of $^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.
$OSNAME
$^O The name of the operating system under which this copy of Perl was built, as determined during the
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 perlipc 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 5.8.0 from immediate (also known as "unsafe")
to deferred, also known as "safe signals". See perlipc 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__".
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 perlfunc, "warn" in perlfunc, "eval" in perlfunc, and warnings for additional
information.
$BASETIME
$^T The time at which the program began running, in seconds since the epoch (beginning of 1970). The
values returned by the -M, -A, and -C filetests are based on this value.
$PERL_VERSION
$^V The revision, version, and subversion of the Perl interpreter, represented as a "version" object.
This variable first appeared in perl 5.6.0; earlier versions of perl will see an undefined value.
Before perl 5.10.0 $^V was represented as a v-string.
$^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
To convert $^V into its string representation use "sprintf()"'s "%vd" conversion:
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 an older representation of the Perl version.
This variable was added in Perl 5.6.
Mnemonic: use ^V for Version Control.
${^WIN32_SLOPPY_STAT}
If this variable is set to a true value, then "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.
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;
}
Variables related to regular expressions
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" }
{
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
Due to an unfortunate accident of Perl's implementation, "use English" imposes a considerable performance
penalty on all regular expression matches in a program because it uses the "$`", $&, and "$'", regardless of
whether they occur in the scope of "use English". For that reason, saying "use English" in libraries is
strongly discouraged unless you import it without the match variables:
use English '-no_match_vars'
The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you find uses of these problematic match
variables in your code.
Since Perl 5.10, you can use the "/p" match operator flag and the "${^PREMATCH}", "${^MATCH}", and
"${^POSTMATCH}" variables instead so you only suffer the performance penalties.
$<digits> ($1, $2, ...)
Contains the subpattern from the corresponding set of capturing parentheses from the last successful
pattern match, not counting patterns matched in nested blocks that have been exited already.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
$MATCH
$& The string matched by the last successful pattern match (not counting any matches hidden within a
BLOCK or "eval()" enclosed by the current BLOCK).
The use of this variable anywhere in a program imposes a considerable performance penalty on all
regular expression matches. To avoid this penalty, you can extract the same substring by using "@-".
Starting with Perl 5.10, you can use the "/p" match flag and the "${^MATCH}" variable to do the same
thing for particular match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: like "&" in some editors.
${^MATCH}
This is similar to $& ($MATCH) except that it does not incur the performance penalty associated with
that variable, and is only guaranteed to return a defined value when the pattern was compiled or
executed with the "/p" modifier.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
$PREMATCH
$` The string preceding whatever was matched by the last successful pattern match, not counting any
matches hidden within a BLOCK or "eval" enclosed by the current BLOCK.
executed with the "/p" modifier.
This variable was added in Perl 5.10
This variable is read-only and dynamically-scoped.
$POSTMATCH
$' The string following whatever was matched by the last successful pattern match (not counting any
matches hidden within a BLOCK or "eval()" enclosed by the current BLOCK). Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
The use of this variable anywhere in a program imposes a considerable performance penalty on all
regular expression matches. To avoid this penalty, you can extract the same substring by using "@-".
Starting with Perl 5.10, you can use the "/p" match flag and the "${^POSTMATCH}" variable to do the
same thing for particular match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: "'" often follows a quoted string.
${^POSTMATCH}
This is similar to "$'" ($POSTMATCH) except that it does not incur the performance penalty associated
with that variable, and is only guaranteed to return a defined value when the pattern was compiled or
executed with the "/p" modifier.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
$LAST_PAREN_MATCH
$+ The text matched by the last bracket of the last successful search pattern. 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.
$LAST_SUBMATCH_RESULT
$^N The text matched by the used group most-recently closed (i.e. the group with the rightmost closing
parenthesis) of the last successful search pattern.
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 5.6.
%LAST_PAREN_MATCH
%+ Similar to "@+", 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).
The underlying behaviour of "%+" is provided by the Tie::Hash::NamedCapture 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 5.10.
This variable is read-only and dynamically-scoped.
@LAST_MATCH_START
@- "$-[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 5.6.
%LAST_MATCH_START
%- Similar to "%+", 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.
}
}
}
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 Tie::Hash::NamedCapture 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 5.10
This variable is read-only and dynamically-scoped.
$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful "(?{ code })" regular expression assertion (see
perlre). May be written to.
This variable was added in Perl 5.005.
${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags. Set to 0 for no debug output even when the "re
'debug'" module is loaded. See re for details.
This variable was added in Perl 5.10.
${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how much memory they utilize. This value by
default is 65536 which corresponds to a 512kB temporary cache. Set this to a higher value to trade
memory for speed when matching large alternations. Set it to a lower value if you want the
optimisations to be as conservative of memory as possible but still occur, and set it to a negative
value to prevent the optimisation and conserve the most memory. Under normal situations this variable
should be of no interest to you.
This variable was added in Perl 5.10.
Variables related to filehandles
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
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"
$ARGV Contains the name of the current file when reading from "<>".
@ARGV The array @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 The special filehandle that iterates over command-line filenames in @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.
ARGVOUT The special filehandle that points to the currently open output file when doing edit-in-place
processing with -i. Useful when you have to do a lot of inserting and don't want to keep modifying
$_. See perlrun for the -i switch.
Handle->output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator. If defined, this value is printed between each of
print's arguments. Default is "undef".
Mnemonic: what is printed when there is a "," in your print statement.
HANDLE->input_line_number( EXPR )
$INPUT_LINE_NUMBER
$NR
$. Current line number for the last filehandle accessed.
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 perlop. Because "<>" never does an
explicit close, line numbers increase across "ARGV" files (but see examples in "eof" in perlfunc).
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.
HANDLE->input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR
$RS
$/ The input record separator, newline by default. This influences Perl's idea of what a "line" is.
Works like awk's RS variable, including treating empty lines as a terminator if set to the null string
(an empty line cannot contain any spaces or tabs). You may set it to a multi-character string to
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. 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 bytes from FILE. 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 will cause reading in the (rest of the)
whole file.
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.
If you perform a record read on a FILE with an encoding layer such as ":encoding(latin1)" or ":utf8",
you may get an invalid string as a result, may leave the FILE positioned between characters in the
stream and may not be reading the number of bytes from the underlying file that you specified. This
behaviour may change without warning in a future version of perl.
See also "Newlines" in perlport. Also see "$.".
Mnemonic: / delimits line boundaries when quoting poetry.
Handle->output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print operator. If defined, this value is printed after the last
of print's arguments. Default is "undef".
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.
HANDLE->autoflush( EXPR )
$OUTPUT_AUTOFLUSH
$| If set to nonzero, forces a flush right away and after every write or print on the currently selected
output channel. Default is 0 (regardless of whether the channel is really buffered by the system or
not; $| 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 perlfunc for that. See "select" in perlfunc on how to select the output
channel. See also IO::Handle.
Mnemonic: when you want your pipes to be piping hot.
Variables related to formats
The special variables for formats are a subset of those for filehandles. See perlform for more information
about Perl's formats.
$% The current page number of the currently selected output channel.
Mnemonic: "%" is page number in nroff.
HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the currently selected output channel.
Mnemonic: lines_on_page - lines_printed.
Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string may be broken to fill continuation fields (starting
with "^") in a format. The default is " \n-", to break on a space, newline, or a hyphen.
Mnemonic: a "colon" in poetry is a part of a line.
HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
$= The current page length (printable lines) of the currently selected output channel. The default is
60.
Mnemonic: = has horizontal lines.
HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME
$^ The name of the current top-of-page format for the currently selected output channel. The default is
the name of the filehandle with "_TOP" appended. For example, the default format top name for the
"STDOUT" filehandle is "STDOUT_TOP".
Mnemonic: points to top of page.
HANDLE->format_name(EXPR)
$FORMAT_NAME
$~ The name of the current report format for the currently selected output channel. The default format
name is the same as the filehandle name. For example, the default format name for the "STDOUT"
filehandle is just "STDOUT".
Mnemonic: brother to $^.
Error Variables
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: $?, $!";
};
Finally, $? may be set to 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 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 $?.
${^CHILD_ERROR_NATIVE}
The native status returned by the last pipe close, backtick ("``") 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 POSIX 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 5.8.9.
$EXTENDED_OS_ERROR
$^E Error information specific to the current operating system. At the moment, this differs from $! 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.
$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter.
$^S State
--------- -------------------
undef Parsing module/eval
true (1) Executing an eval
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
the $^H and "%^H" variables. The exact values are considered internal to the warnings pragma and may
change between versions of Perl.
This variable was added in Perl 5.6.
$OS_ERROR
$ERRNO
$! When referenced, $! 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?
%OS_ERROR
%ERRNO
%! Each element of "%!" 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). To check if a particular key is meaningful on your system, use "exists
$!{the_key}"; for a list of legal keys, use "keys %!". See Errno for more information, and also see
"$!".
This variable was added in Perl 5.005.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick ("``") 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.
the default emulation of POSIX status; see "$?" in perlvms for details.
Mnemonic: similar to sh and ksh.
$EVAL_ERROR
$@ The Perl syntax error message from the last "eval()" operator. If $@ is the null string, the last
"eval()" parsed and executed correctly (although the operations you invoked may have failed in the
normal fashion).
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 syntax error "at"?
Variables related to the interpreter state
These variables provide information about the current interpreter state.
$COMPILING
$^C The current value of the flag associated with the -c switch. Mainly of use with -MO=... to allow code
to alter its behavior when being compiled, such as for example to "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 5.6.
$DEBUGGING
$^D The current value of the debugging flags. May be read or set. Like its command-line equivalent, you
can use numeric or symbolic values, eg "$^D = 10" or "$^D = "st"".
Mnemonic: value of -D switch.
${^ENCODING}
The object reference to the "Encode" object that is used to convert the source code to Unicode.
Thanks to this variable your Perl script does not have to be written in UTF-8. Default is undef. The
direct manipulation of this variable is highly discouraged.
This variable was added in Perl 5.8.2.
${^GLOBAL_PHASE}
The current phase of the perl interpreter.
Possible values are:
CONSTRUCT
The "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.
START This is the global compile-time. That includes, basically, every "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}".
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.
$^H WARNING: This variable is strictly for internal use only. Its availability, behavior, and contents
are subject to change without notice.
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 The "%^H" hash provides the same scoping semantic as $^H. This makes it useful for implementation of
lexically scoped pragmas. See perlpragma.
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 5.6.
${^OPEN}
An internal variable used by PerlIO. A string in two parts, separated by a "\0" byte, the first part
describes the input layers, the second part describes the output layers.
This variable was added in Perl 5.8.0.
$PERLDB
$^P The internal variable for debugging support. The meanings of the various bits are subject to change,
but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging. Causes "DB::DB()" subroutine to be called for each statement executed.
Also causes saving source code lines (like 0x400).
0x100 Provide informative "file" names for evals based on the place they were compiled.
0x200 Provide informative names to anonymous subroutines based on the place they were compiled.
0x400 Save source code lines into "@{"_<$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 perldebguts.
${^TAINT}
Reflects if taint mode is on or off. 1 for on (the program was run with -T), 0 for off, -1 when only
taint warnings are enabled (i.e. with -t or -TU).
This variable is read-only.
This variable was added in Perl 5.8.
${^UNICODE}
Reflects certain Unicode settings of Perl. See perlrun documentation for the "-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 5.8.2.
${^UTF8CACHE}
This variable controls the state of the internal UTF-8 offset caching code. 1 for on (the default), 0
for off, -1 to debug the caching code by checking all its results against linear scans, and panicking
on any discrepancy.
This variable was added in Perl 5.8.9.
${^UTF8LOCALE}
This variable indicates whether a UTF-8 locale was detected by perl at startup. This information is
used by perl when it's in adjust-utf8ness-to-locale mode (as when run with the "-CL" command-line
switch); see perlrun for more info on this.
This variable was added in Perl 5.8.8.
Deprecated and removed variables
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 perldiag for details about error messages.
$OFMT
$# $# was a variable that could be used to format printed numbers. After a deprecation cycle, its magic
was removed in Perl 5.10 and using it now triggers a warning: "$# 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.
$ARRAY_BASE
$[ This variable stores the index of the first element in an array, and of the first character in a
substring. The default is 0, but you could theoretically set it to 1 to make Perl behave more like
awk (or Fortran) when subscripting and when evaluating the 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 5.10, assignment to $[ could be seen from outer lexical scopes in the same file, unlike
other compile-time directives (such as strict). Using local() on it would bind its value strictly to
a lexical block. Now it is always lexically scoped.
As of Perl 5.16, it is implemented by the arybase module. See arybase for more details on its
behaviour.
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 5.12.
$OLD_PERL_VERSION
$] See "$^V" for a more modern representation of the Perl version that allows accurate string
comparisons.
The version + patchlevel / 1000 of the Perl interpreter. This variable can be used to determine
whether the Perl interpreter executing a script is in the right range of versions:
warn "No checksumming!\n" if $] < 3.019;
The floating point representation can sometimes lead to inaccurate numeric comparisons.
See also the documentation of "use VERSION" and "require VERSION" for a convenient way to fail if the
running Perl interpreter is too old.
Mnemonic: Is this version of perl in the right bracket?
perl v5.16.3 2013-03-04 PERLVAR(1)
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