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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
b0c22438 7=head2 The Syntax of Variable Names
8
241a59d9 9Variable names in Perl can have several formats. Usually, they
b0c22438 10must begin with a letter or underscore, in which case they can be
11arbitrarily long (up to an internal limit of 251 characters) and
12may contain letters, digits, underscores, or the special sequence
241a59d9 13C<::> or C<'>. In this case, the part before the last C<::> or
b0c22438 14C<'> is taken to be a I<package qualifier>; see L<perlmod>.
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15A Unicode letter that is not ASCII is not considered to be a letter
16unless S<C<"use utf8">> is in effect, and somewhat more complicated
17rules apply; see L<perldata/Identifier parsing> for details.
b0c22438 18
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19Perl variable names may also be a sequence of digits, a single
20punctuation character, or the two-character sequence: C<^> (caret or
21CIRCUMFLEX ACCENT) followed by any one of the characters C<[][A-Z^_?\]>.
22These names are all reserved for
b0c22438 23special uses by Perl; for example, the all-digits names are used
24to hold data captured by backreferences after a regular expression
ce4793f1 25match.
b0c22438 26
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27Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
28preceded by a caret. These must all be written in the form C<${^Foo}>;
29the braces are not optional. C<${^Foo}> denotes the scalar variable
30whose name is considered to be a control-C<F> followed by two C<o>'s.
31These variables are
b0c22438 32reserved for future special uses by Perl, except for the ones that
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33begin with C<^_> (caret-underscore). No
34name that begins with C<^_> will acquire a special
b0c22438 35meaning in any future version of Perl; such names may therefore be
241a59d9 36used safely in programs. C<$^_> itself, however, I<is> reserved.
b0c22438 37
ce4793f1 38Perl identifiers that begin with digits or
b0c22438 39punctuation characters are exempt from the effects of the C<package>
40declaration and are always forced to be in package C<main>; they are
241a59d9 41also exempt from C<strict 'vars'> errors. A few other names are also
b0c22438 42exempt in these ways:
43
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44 ENV STDIN
45 INC STDOUT
46 ARGV STDERR
47 ARGVOUT
48 SIG
b0c22438 49
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
53
54=head1 SPECIAL VARIABLES
a0d0e21e 55
241a59d9 56The following names have special meaning to Perl. Most punctuation
0b9346e6 57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
9548c15c 60 use English;
a0d0e21e 61
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62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
1e7d0944 64borrowed from B<awk>. For more info, please see L<English>.
a1ce9542 65
241a59d9 66Before you continue, note the sort order for variables. In general, we
0b9346e6 67first list the variables in case-insensitive, almost-lexigraphical
68order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
69or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
70For variables with the same identifier, we list it in order of scalar,
71array, hash, and bareword.
a1ce9542 72
b0c22438 73=head2 General Variables
a0d0e21e 74
84dabc03 75=over 8
76
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77=item $ARG
78
79=item $_
a054c801 80X<$_> X<$ARG>
a0d0e21e 81
241a59d9 82The default input and pattern-searching space. The following pairs are
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83equivalent:
84
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85 while (<>) {...} # equivalent only in while!
86 while (defined($_ = <>)) {...}
a0d0e21e 87
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88 /^Subject:/
89 $_ =~ /^Subject:/
a0d0e21e 90
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91 tr/a-z/A-Z/
92 $_ =~ tr/a-z/A-Z/
a0d0e21e 93
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94 chomp
95 chomp($_)
a0d0e21e 96
0b9346e6 97Here are the places where Perl will assume C<$_> even if you don't use it:
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98
99=over 3
100
101=item *
102
84dabc03 103The following functions use C<$_> as a default argument:
db1511c8 104
f61f53cc 105abs, alarm, chomp, chop, chr, chroot,
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106cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
107lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
b0169937 108quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
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109rmdir, say, sin, split (for its second
110argument), sqrt, stat, study, uc, ucfirst,
b0169937 111unlink, unpack.
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112
113=item *
114
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115All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
116See L<perlfunc/-X>
117
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118=item *
119
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120The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
121when used without an C<=~> operator.
cb1a09d0 122
54310121 123=item *
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124
125The default iterator variable in a C<foreach> loop if no other
126variable is supplied.
127
54310121 128=item *
cb1a09d0 129
b0c22438 130The implicit iterator variable in the C<grep()> and C<map()> functions.
cb1a09d0 131
54310121 132=item *
cb1a09d0 133
b0c22438 134The implicit variable of C<given()>.
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135
136=item *
137
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138The default place to put the next value or input record
139when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
cb1a09d0 140operation's result is tested by itself as the sole criterion of a C<while>
241a59d9 141test. Outside a C<while> test, this will not happen.
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142
143=back
144
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145C<$_> is by default a global variable. However, as
146of perl v5.10.0, you can use a lexical version of
241a59d9 147C<$_> by declaring it in a file or in a block with C<my>. Moreover,
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148declaring C<our $_> restores the global C<$_> in the current scope. Though
149this seemed like a good idea at the time it was introduced, lexical C<$_>
150actually causes more problems than it solves. If you call a function that
151expects to be passed information via C<$_>, it may or may not work,
152depending on how the function is written, there not being any easy way to
153solve this. Just avoid lexical C<$_>, unless you are feeling particularly
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154masochistic. For this reason lexical C<$_> is still experimental and will
155produce a warning unless warnings have been disabled. As with other
156experimental features, the behavior of lexical C<$_> is subject to change
157without notice, including change into a fatal error.
59f00321 158
b0c22438 159Mnemonic: underline is understood in certain operations.
a0d0e21e 160
0b9346e6 161=item @ARG
cde0cee5 162
0b9346e6 163=item @_
164X<@_> X<@ARG>
a0d0e21e 165
0b9346e6 166Within a subroutine the array C<@_> contains the parameters passed to
241a59d9 167that subroutine. Inside a subroutine, C<@_> is the default array for
256ca3d3 168the array operators C<pop> and C<shift>.
a0d0e21e 169
0b9346e6 170See L<perlsub>.
a0d0e21e 171
1311257d 172=item $LIST_SEPARATOR
173
174=item $"
175X<$"> X<$LIST_SEPARATOR>
176
69520822 177When an array or an array slice is interpolated into a double-quoted
178string or a similar context such as C</.../>, its elements are
241a59d9 179separated by this value. Default is a space. For example, this:
69520822 180
9548c15c 181 print "The array is: @array\n";
69520822 182
183is equivalent to this:
184
9548c15c 185 print "The array is: " . join($", @array) . "\n";
69520822 186
187Mnemonic: works in double-quoted context.
1311257d 188
b0c22438 189=item $PROCESS_ID
cde0cee5 190
b0c22438 191=item $PID
a0d0e21e 192
b0c22438 193=item $$
194X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 195
241a59d9 196The process number of the Perl running this script. Though you I<can> set
4a904372 197this variable, doing so is generally discouraged, although it can be
241a59d9 198invaluable for some testing purposes. It will be reset automatically
b0c22438 199across C<fork()> calls.
a0d0e21e 200
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201Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
202would emulate POSIX semantics on Linux systems using LinuxThreads, a
203partial implementation of POSIX Threads that has since been superseded
204by the Native POSIX Thread Library (NPTL).
205
e3f68f70 206LinuxThreads is now obsolete on Linux, and caching C<getpid()>
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207like this made embedding perl unnecessarily complex (since you'd have
208to manually update the value of $$), so now C<$$> and C<getppid()>
209will always return the same values as the underlying C library.
210
211Debian GNU/kFreeBSD systems also used LinuxThreads up until and
212including the 6.0 release, but after that moved to FreeBSD thread
213semantics, which are POSIX-like.
214
215To see if your system is affected by this discrepancy check if
216C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
1e7d0944 217value. NTPL threads preserve the POSIX semantics.
a0d0e21e 218
b0c22438 219Mnemonic: same as shells.
ad83b128 220
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221=item $PROGRAM_NAME
222
223=item $0
224X<$0> X<$PROGRAM_NAME>
225
226Contains the name of the program being executed.
227
228On some (but not all) operating systems assigning to C<$0> modifies
241a59d9 229the argument area that the C<ps> program sees. On some platforms you
66d7055b 230may have to use special C<ps> options or a different C<ps> to see the
241a59d9 231changes. Modifying the C<$0> is more useful as a way of indicating the
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232current program state than it is for hiding the program you're
233running.
234
235Note that there are platform-specific limitations on the maximum
241a59d9 236length of C<$0>. In the most extreme case it may be limited to the
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237space occupied by the original C<$0>.
238
239In some platforms there may be arbitrary amount of padding, for
240example space characters, after the modified name as shown by C<ps>.
241In some platforms this padding may extend all the way to the original
242length of the argument area, no matter what you do (this is the case
243for example with Linux 2.2).
244
245Note for BSD users: setting C<$0> does not completely remove "perl"
241a59d9 246from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
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247result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
248and the " (perl)" suffix are shown depends on your exact BSD variant
241a59d9 249and version). This is an operating system feature, Perl cannot help it.
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250
251In multithreaded scripts Perl coordinates the threads so that any
252thread may modify its copy of the C<$0> and the change becomes visible
241a59d9 253to ps(1) (assuming the operating system plays along). Note that
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254the view of C<$0> the other threads have will not change since they
255have their own copies of it.
256
257If the program has been given to perl via the switches C<-e> or C<-E>,
258C<$0> will contain the string C<"-e">.
259
60cf4914 260On Linux as of perl v5.14.0 the legacy process name will be set with
66d7055b 261C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
241a59d9 262perl has done since version 4.000. Now system utilities that read the
66d7055b 263legacy process name such as ps, top and killall will recognize the
241a59d9 264name you set when assigning to C<$0>. The string you supply will be
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265cut off at 16 bytes, this is a limitation imposed by Linux.
266
267Mnemonic: same as B<sh> and B<ksh>.
268
b0c22438 269=item $REAL_GROUP_ID
a01268b5 270
b0c22438 271=item $GID
a01268b5 272
b0c22438 273=item $(
274X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 275
241a59d9 276The real gid of this process. If you are on a machine that supports
b0c22438 277membership in multiple groups simultaneously, gives a space separated
241a59d9 278list of groups you are in. The first number is the one returned by
b0c22438 279C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
280the same as the first number.
a01268b5 281
b0c22438 282However, a value assigned to C<$(> must be a single number used to
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283set the real gid. So the value given by C<$(> should I<not> be assigned
284back to C<$(> without being forced numeric, such as by adding zero. Note
b0c22438 285that this is different to the effective gid (C<$)>) which does take a
286list.
fe307981 287
b0c22438 288You can change both the real gid and the effective gid at the same
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289time by using C<POSIX::setgid()>. Changes
290to C<$(> require a check to C<$!>
b0c22438 291to detect any possible errors after an attempted change.
6cef1e77 292
241a59d9 293Mnemonic: parentheses are used to I<group> things. The real gid is the
b0c22438 294group you I<left>, if you're running setgid.
6cef1e77 295
b0c22438 296=item $EFFECTIVE_GROUP_ID
8e08999f 297
b0c22438 298=item $EGID
81714fb9 299
b0c22438 300=item $)
301X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
81714fb9 302
241a59d9 303The effective gid of this process. If you are on a machine that
b0c22438 304supports membership in multiple groups simultaneously, gives a space
241a59d9 305separated list of groups you are in. The first number is the one
b0c22438 306returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
307one of which may be the same as the first number.
81714fb9 308
b0c22438 309Similarly, a value assigned to C<$)> must also be a space-separated
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310list of numbers. The first number sets the effective gid, and
311the rest (if any) are passed to C<setgroups()>. To get the effect of an
b0c22438 312empty list for C<setgroups()>, just repeat the new effective gid; that is,
313to force an effective gid of 5 and an effectively empty C<setgroups()>
314list, say C< $) = "5 5" >.
81714fb9 315
b0c22438 316You can change both the effective gid and the real gid at the same
317time by using C<POSIX::setgid()> (use only a single numeric argument).
318Changes to C<$)> require a check to C<$!> to detect any possible errors
319after an attempted change.
44a2ac75 320
b0c22438 321C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
241a59d9 322machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
b0c22438 323and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 324
241a59d9 325Mnemonic: parentheses are used to I<group> things. The effective gid
b0c22438 326is the group that's I<right> for you, if you're running setgid.
44a2ac75 327
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328=item $REAL_USER_ID
329
330=item $UID
331
332=item $<
333X<< $< >> X<$UID> X<$REAL_USER_ID>
334
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335The real uid of this process. You can change both the real uid and the
336effective uid at the same time by using C<POSIX::setuid()>. Since
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337changes to C<< $< >> require a system call, check C<$!> after a change
338attempt to detect any possible errors.
339
340Mnemonic: it's the uid you came I<from>, if you're running setuid.
341
342=item $EFFECTIVE_USER_ID
343
344=item $EUID
345
346=item $>
347X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
348
241a59d9 349The effective uid of this process. For example:
c82f2f4e 350
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351 $< = $>; # set real to effective uid
352 ($<,$>) = ($>,$<); # swap real and effective uids
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353
354You can change both the effective uid and the real uid at the same
241a59d9 355time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
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356to C<$!> to detect any possible errors after an attempted change.
357
358C<< $< >> and C<< $> >> can be swapped only on machines
359supporting C<setreuid()>.
360
361Mnemonic: it's the uid you went I<to>, if you're running setuid.
362
0b9346e6 363=item $SUBSCRIPT_SEPARATOR
364
365=item $SUBSEP
366
367=item $;
368X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
369
241a59d9 370The subscript separator for multidimensional array emulation. If you
0b9346e6 371refer to a hash element as
372
592708b4 373 $foo{$x,$y,$z}
0b9346e6 374
375it really means
376
592708b4 377 $foo{join($;, $x, $y, $z)}
0b9346e6 378
379But don't put
380
592708b4 381 @foo{$x,$y,$z} # a slice--note the @
0b9346e6 382
383which means
384
592708b4 385 ($foo{$x},$foo{$y},$foo{$z})
0b9346e6 386
241a59d9 387Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 388binary data there might not be any safe value for C<$;>.
389
390Consider using "real" multidimensional arrays as described
391in L<perllol>.
392
393Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
394
0b9346e6 395=item $a
396
397=item $b
398X<$a> X<$b>
399
400Special package variables when using C<sort()>, see L<perlfunc/sort>.
401Because of this specialness C<$a> and C<$b> don't need to be declared
402(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
241a59d9 403pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
0b9346e6 404be able to use them in the C<sort()> comparison block or function.
405
0b9346e6 406=item %ENV
407X<%ENV>
408
241a59d9 409The hash C<%ENV> contains your current environment. Setting a
0b9346e6 410value in C<ENV> changes the environment for any child processes
411you subsequently C<fork()> off.
412
32e006ac 413As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
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414
415 my $foo = 1;
416 $ENV{'bar'} = \$foo;
417 if( ref $ENV{'bar'} ) {
32e006ac 418 say "Pre 5.18.0 Behaviour";
a5effbbc 419 } else {
32e006ac 420 say "Post 5.18.0 Behaviour";
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421 }
422
423Previously, only child processes received stringified values:
424
425 my $foo = 1;
426 $ENV{'bar'} = \$foo;
427
428 # Always printed 'non ref'
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429 system($^X, '-e',
430 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
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431
432This happens because you can't really share arbitrary data structures with
433foreign processes.
434
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435=item $OLD_PERL_VERSION
436
4ad0ecd4 437=item $]
b77ebf74 438X<$]> X<$OLD_PERL_VERSION>
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439
440The revision, version, and subversion of the Perl interpreter, represented
441as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
442is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
443
444This variable can be used to determine whether the Perl interpreter
445executing a script is in the right range of versions:
446
447 warn "No PerlIO!\n" if $] lt '5.008';
448
449When comparing C<$]>, string comparison operators are B<highly
450recommended>. The inherent limitations of binary floating point
451representation can sometimes lead to incorrect comparisons for some
452numbers on some architectures.
453
454See also the documentation of C<use VERSION> and C<require VERSION>
455for a convenient way to fail if the running Perl interpreter is too old.
456
457See L</$^V> for a representation of the Perl version as a L<version>
458object, which allows more flexible string comparisons.
459
460The main advantage of C<$]> over C<$^V> is that it works the same on any
461version of Perl. The disadvantages are that it can't easily be compared
462to versions in other formats (e.g. literal v-strings, "v1.2.3" or
463version objects) and numeric comparisons can occasionally fail; it's good
464for string literal version checks and bad for comparing to a variable
465that hasn't been sanity-checked.
466
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467The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
468reasons but its use is discouraged. (If your reason to use C<$]> is to
469run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
470be self-defeating.)
4ad0ecd4 471
9fcc2314 472Mnemonic: Is this version of perl in the right bracket?
83c1fffe 473
b0c22438 474=item $SYSTEM_FD_MAX
5b2b9c68 475
b0c22438 476=item $^F
477X<$^F> X<$SYSTEM_FD_MAX>
5b2b9c68 478
241a59d9 479The maximum system file descriptor, ordinarily 2. System file
b0c22438 480descriptors are passed to C<exec()>ed processes, while higher file
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481descriptors are not. Also, during an
482C<open()>, system file descriptors are
b0c22438 483preserved even if the C<open()> fails (ordinary file descriptors are
241a59d9 484closed before the C<open()> is attempted). The close-on-exec
b0c22438 485status of a file descriptor will be decided according to the value of
486C<$^F> when the corresponding file, pipe, or socket was opened, not the
487time of the C<exec()>.
5b2b9c68 488
0b9346e6 489=item @F
490X<@F>
491
492The array C<@F> contains the fields of each line read in when autosplit
241a59d9 493mode is turned on. See L<perlrun> for the B<-a> switch. This array
0b9346e6 494is package-specific, and must be declared or given a full package name
495if not in package main when running under C<strict 'vars'>.
496
0b9346e6 497=item @INC
498X<@INC>
499
500The array C<@INC> contains the list of places that the C<do EXPR>,
241a59d9 501C<require>, or C<use> constructs look for their library files. It
0b9346e6 502initially consists of the arguments to any B<-I> command-line
503switches, followed by the default Perl library, probably
504F</usr/local/lib/perl>, followed by ".", to represent the current
241a59d9 505directory. ("." will not be appended if taint checks are enabled,
0b9346e6 506either by C<-T> or by C<-t>.) If you need to modify this at runtime,
507you should use the C<use lib> pragma to get the machine-dependent
508library properly loaded also:
509
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510 use lib '/mypath/libdir/';
511 use SomeMod;
0b9346e6 512
513You can also insert hooks into the file inclusion system by putting Perl
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514code directly into C<@INC>. Those hooks may be subroutine references,
515array references or blessed objects. See L<perlfunc/require> for details.
0b9346e6 516
517=item %INC
518X<%INC>
519
520The hash C<%INC> contains entries for each filename included via the
241a59d9 521C<do>, C<require>, or C<use> operators. The key is the filename
0b9346e6 522you specified (with module names converted to pathnames), and the
241a59d9 523value is the location of the file found. The C<require>
0b9346e6 524operator uses this hash to determine whether a particular file has
525already been included.
526
527If the file was loaded via a hook (e.g. a subroutine reference, see
528L<perlfunc/require> for a description of these hooks), this hook is
241a59d9 529by default inserted into C<%INC> in place of a filename. Note, however,
0b9346e6 530that the hook may have set the C<%INC> entry by itself to provide some more
531specific info.
532
b0c22438 533=item $INPLACE_EDIT
a0d0e21e 534
b0c22438 535=item $^I
536X<$^I> X<$INPLACE_EDIT>
a0d0e21e 537
241a59d9 538The current value of the inplace-edit extension. Use C<undef> to disable
b0c22438 539inplace editing.
a0d0e21e 540
b0c22438 541Mnemonic: value of B<-i> switch.
a0d0e21e 542
b0c22438 543=item $^M
544X<$^M>
a0d0e21e 545
b0c22438 546By default, running out of memory is an untrappable, fatal error.
547However, if suitably built, Perl can use the contents of C<$^M>
241a59d9 548as an emergency memory pool after C<die()>ing. Suppose that your Perl
b0c22438 549were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
550Then
a0d0e21e 551
9548c15c 552 $^M = 'a' x (1 << 16);
a0d0e21e 553
241a59d9 554would allocate a 64K buffer for use in an emergency. See the
b0c22438 555F<INSTALL> file in the Perl distribution for information on how to
241a59d9 556add custom C compilation flags when compiling perl. To discourage casual
b0c22438 557use of this advanced feature, there is no L<English|English> long name for
558this variable.
a0d0e21e 559
b0c22438 560This variable was added in Perl 5.004.
a0d0e21e 561
b0c22438 562=item $OSNAME
a0d0e21e 563
b0c22438 564=item $^O
565X<$^O> X<$OSNAME>
a0d0e21e 566
b0c22438 567The name of the operating system under which this copy of Perl was
241a59d9 568built, as determined during the configuration process. For examples
b0c22438 569see L<perlport/PLATFORMS>.
a0d0e21e 570
241a59d9 571The value is identical to C<$Config{'osname'}>. See also L<Config>
b0c22438 572and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 573
b0c22438 574In Windows platforms, C<$^O> is not very helpful: since it is always
575C<MSWin32>, it doesn't tell the difference between
241a59d9 57695/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
b0c22438 577Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
578between the variants.
a0d0e21e 579
b0c22438 580This variable was added in Perl 5.003.
a0d0e21e 581
1fa81471
DR
582=item %SIG
583X<%SIG>
a0d0e21e 584
241a59d9 585The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 586
9548c15c
FC
587 sub handler { # 1st argument is signal name
588 my($sig) = @_;
589 print "Caught a SIG$sig--shutting down\n";
590 close(LOG);
591 exit(0);
592 }
a0d0e21e 593
9548c15c
FC
594 $SIG{'INT'} = \&handler;
595 $SIG{'QUIT'} = \&handler;
596 ...
597 $SIG{'INT'} = 'DEFAULT'; # restore default action
598 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 599
1fa81471 600Using a value of C<'IGNORE'> usually has the effect of ignoring the
241a59d9 601signal, except for the C<CHLD> signal. See L<perlipc> for more about
1fa81471 602this special case.
a0d0e21e 603
1fa81471 604Here are some other examples:
a0d0e21e 605
9548c15c
FC
606 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
607 # recommended)
608 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
609 # Plumber
610 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
611 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
612 # return??
a0d0e21e 613
1fa81471
DR
614Be sure not to use a bareword as the name of a signal handler,
615lest you inadvertently call it.
a0d0e21e 616
1fa81471 617If your system has the C<sigaction()> function then signal handlers
241a59d9 618are installed using it. This means you get reliable signal handling.
a0d0e21e 619
60cf4914 620The default delivery policy of signals changed in Perl v5.8.0 from
1fa81471 621immediate (also known as "unsafe") to deferred, also known as "safe
241a59d9 622signals". See L<perlipc> for more information.
a0d0e21e 623
241a59d9 624Certain internal hooks can be also set using the C<%SIG> hash. The
1fa81471 625routine indicated by C<$SIG{__WARN__}> is called when a warning
241a59d9
FC
626message is about to be printed. The warning message is passed as the
627first argument. The presence of a C<__WARN__> hook causes the
628ordinary printing of warnings to C<STDERR> to be suppressed. You can
1fa81471
DR
629use this to save warnings in a variable, or turn warnings into fatal
630errors, like this:
a0d0e21e 631
9548c15c
FC
632 local $SIG{__WARN__} = sub { die $_[0] };
633 eval $proggie;
a8f8344d 634
b0c22438 635As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
636disable warnings using the empty subroutine:
f86702cc 637
9548c15c 638 local $SIG{__WARN__} = sub {};
55602bd2 639
b0c22438 640The routine indicated by C<$SIG{__DIE__}> is called when a fatal
241a59d9
FC
641exception is about to be thrown. The error message is passed as the
642first argument. When a C<__DIE__> hook routine returns, the exception
b0c22438 643processing continues as it would have in the absence of the hook,
c94b42ea
DM
644unless the hook routine itself exits via a C<goto &sub>, a loop exit,
645or a C<die()>. The C<__DIE__> handler is explicitly disabled during
646the call, so that you can die from a C<__DIE__> handler. Similarly
647for C<__WARN__>.
e5218da5 648
b0c22438 649Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
241a59d9 650even inside an C<eval()>. Do not use this to rewrite a pending
b0c22438 651exception in C<$@>, or as a bizarre substitute for overriding
241a59d9 652C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
b0c22438 653in a future release so that C<$SIG{__DIE__}> is only called if your
241a59d9 654program is about to exit, as was the original intent. Any other use is
b0c22438 655deprecated.
656
657C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
241a59d9 658may be called to report (probable) errors found by the parser. In such
b0c22438 659a case the parser may be in inconsistent state, so any attempt to
660evaluate Perl code from such a handler will probably result in a
241a59d9 661segfault. This means that warnings or errors that result from parsing
b0c22438 662Perl should be used with extreme caution, like this:
e5218da5 663
9548c15c
FC
664 require Carp if defined $^S;
665 Carp::confess("Something wrong") if defined &Carp::confess;
666 die "Something wrong, but could not load Carp to give "
667 . "backtrace...\n\t"
668 . "To see backtrace try starting Perl with -MCarp switch";
e5218da5 669
b0c22438 670Here the first line will load C<Carp> I<unless> it is the parser who
241a59d9
FC
671called the handler. The second line will print backtrace and die if
672C<Carp> was available. The third line will be executed only if C<Carp> was
b0c22438 673not available.
0a378802 674
0b9346e6 675Having to even think about the C<$^S> variable in your exception
241a59d9
FC
676handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
677invites grievous and difficult to track down errors. Avoid it
0b9346e6 678and use an C<END{}> or CORE::GLOBAL::die override instead.
679
b0c22438 680See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
681L<warnings> for additional information.
0a378802 682
b0c22438 683=item $BASETIME
6ab308ee 684
b0c22438 685=item $^T
686X<$^T> X<$BASETIME>
6ab308ee 687
b0c22438 688The time at which the program began running, in seconds since the
241a59d9 689epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
b0c22438 690and B<-C> filetests are based on this value.
a0d0e21e 691
b0c22438 692=item $PERL_VERSION
a0d0e21e 693
b0c22438 694=item $^V
695X<$^V> X<$PERL_VERSION>
a0d0e21e 696
b0c22438 697The revision, version, and subversion of the Perl interpreter,
eb82332c 698represented as a L<version> object.
748a9306 699
60cf4914
BF
700This variable first appeared in perl v5.6.0; earlier versions of perl
701will see an undefined value. Before perl v5.10.0 C<$^V> was represented
f20d3573 702as a v-string rather than a L<version> object.
55602bd2 703
b0c22438 704C<$^V> can be used to determine whether the Perl interpreter executing
241a59d9 705a script is in the right range of versions. For example:
a0d0e21e 706
9548c15c 707 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 708
f20d3573
DG
709While version objects overload stringification, to portably convert
710C<$^V> into its string representation, use C<sprintf()>'s C<"%vd">
711conversion, which works for both v-strings or version objects:
a0d0e21e 712
9548c15c 713 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 714
b0c22438 715See the documentation of C<use VERSION> and C<require VERSION>
716for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 717
eb82332c 718See also C<$]> for a decimal representation of the Perl version.
a0d0e21e 719
f20d3573
DG
720The main advantage of C<$^V> over C<$]> is that, for Perl v5.10.0 or
721later, it overloads operators, allowing easy comparison against other
722version representations (e.g. decimal, literal v-string, "v1.2.3", or
723objects). The disadvantage is that prior to v5.10.0, it was only a
724literal v-string, which can't be easily printed or compared.
a0d0e21e 725
eb82332c 726Mnemonic: use ^V for a version object.
a0d0e21e 727
b0c22438 728=item ${^WIN32_SLOPPY_STAT}
5b442a2a 729X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
a0d0e21e 730
b0c22438 731If this variable is set to a true value, then C<stat()> on Windows will
241a59d9 732not try to open the file. This means that the link count cannot be
b0c22438 733determined and file attributes may be out of date if additional
241a59d9 734hardlinks to the file exist. On the other hand, not opening the file
b0c22438 735is considerably faster, especially for files on network drives.
a0d0e21e 736
b0c22438 737This variable could be set in the F<sitecustomize.pl> file to
738configure the local Perl installation to use "sloppy" C<stat()> by
241a59d9 739default. See the documentation for B<-f> in
b0c22438 740L<perlrun|perlrun/"Command Switches"> for more information about site
741customization.
a0d0e21e 742
60cf4914 743This variable was added in Perl v5.10.0.
a0d0e21e 744
b0c22438 745=item $EXECUTABLE_NAME
a0d0e21e 746
b0c22438 747=item $^X
748X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 749
b0c22438 750The name used to execute the current copy of Perl, from C's
751C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 752
b0c22438 753Depending on the host operating system, the value of C<$^X> may be
754a relative or absolute pathname of the perl program file, or may
755be the string used to invoke perl but not the pathname of the
241a59d9 756perl program file. Also, most operating systems permit invoking
b0c22438 757programs that are not in the PATH environment variable, so there
241a59d9 758is no guarantee that the value of C<$^X> is in PATH. For VMS, the
b0c22438 759value may or may not include a version number.
a0d0e21e 760
b0c22438 761You usually can use the value of C<$^X> to re-invoke an independent
762copy of the same perl that is currently running, e.g.,
a0d0e21e 763
9548c15c 764 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 765
b0c22438 766But recall that not all operating systems support forking or
767capturing of the output of commands, so this complex statement
768may not be portable.
a0d0e21e 769
b0c22438 770It is not safe to use the value of C<$^X> as a path name of a file,
771as some operating systems that have a mandatory suffix on
772executable files do not require use of the suffix when invoking
241a59d9 773a command. To convert the value of C<$^X> to a path name, use the
b0c22438 774following statements:
8cc95fdb 775
9548c15c
FC
776 # Build up a set of file names (not command names).
777 use Config;
778 my $this_perl = $^X;
779 if ($^O ne 'VMS') {
780 $this_perl .= $Config{_exe}
781 unless $this_perl =~ m/$Config{_exe}$/i;
782 }
8cc95fdb 783
b0c22438 784Because many operating systems permit anyone with read access to
785the Perl program file to make a copy of it, patch the copy, and
786then execute the copy, the security-conscious Perl programmer
787should take care to invoke the installed copy of perl, not the
241a59d9 788copy referenced by C<$^X>. The following statements accomplish
b0c22438 789this goal, and produce a pathname that can be invoked as a
790command or referenced as a file.
a043a685 791
9548c15c
FC
792 use Config;
793 my $secure_perl_path = $Config{perlpath};
794 if ($^O ne 'VMS') {
795 $secure_perl_path .= $Config{_exe}
796 unless $secure_perl_path =~ m/$Config{_exe}$/i;
797 }
a0d0e21e 798
b0c22438 799=back
a0d0e21e 800
b0c22438 801=head2 Variables related to regular expressions
802
803Most of the special variables related to regular expressions are side
241a59d9
FC
804effects. Perl sets these variables when it has a successful match, so
805you should check the match result before using them. For instance:
b0c22438 806
9548c15c
FC
807 if( /P(A)TT(ER)N/ ) {
808 print "I found $1 and $2\n";
809 }
b0c22438 810
0b9346e6 811These variables are read-only and dynamically-scoped, unless we note
b0c22438 812otherwise.
813
0b9346e6 814The dynamic nature of the regular expression variables means that
815their value is limited to the block that they are in, as demonstrated
816by this bit of code:
b0c22438 817
9548c15c
FC
818 my $outer = 'Wallace and Grommit';
819 my $inner = 'Mutt and Jeff';
0b9346e6 820
9548c15c 821 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 822
9548c15c 823 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 824
9548c15c
FC
825 {
826 OUTER:
827 show_n() if $outer =~ m/$pattern/;
0b9346e6 828
9548c15c
FC
829 INNER: {
830 show_n() if $inner =~ m/$pattern/;
831 }
0b9346e6 832
9548c15c
FC
833 show_n();
834 }
b0c22438 835
0b9346e6 836The output shows that while in the C<OUTER> block, the values of C<$1>
241a59d9 837and C<$2> are from the match against C<$outer>. Inside the C<INNER>
0b9346e6 838block, the values of C<$1> and C<$2> are from the match against
839C<$inner>, but only until the end of the block (i.e. the dynamic
241a59d9 840scope). After the C<INNER> block completes, the values of C<$1> and
0b9346e6 841C<$2> return to the values for the match against C<$outer> even though
b0c22438 842we have not made another match:
843
9548c15c
FC
844 $1 is Wallace; $2 is Grommit
845 $1 is Mutt; $2 is Jeff
846 $1 is Wallace; $2 is Grommit
a0d0e21e 847
40445027 848=head3 Performance issues
0b9346e6 849
40445027
DM
850Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
851or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
852all subsequent successful pattern matches to make a copy of the matched
853string, in case the code might subsequently access one of those variables.
854This imposed a considerable performance penalty across the whole program,
855so generally the use of these variables has been discouraged.
0b9346e6 856
40445027
DM
857In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
858supply the indices of successful matches. So you could for example do
859this:
860
861 $str =~ /pattern/;
862
863 print $`, $&, $'; # bad: perfomance hit
864
865 print # good: no perfomance hit
866 substr($str, 0, $-[0]),
867 substr($str, $-[0], $+[0]-$-[0]),
868 substr($str, $+[0]);
869
870In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
871C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
872you to suffer the penalties only on patterns marked with C</p>.
873
874In Perl 5.18.0 onwards, perl started noting the presence of each of the
875three variables separately, and only copied that part of the string
876required; so in
877
878 $`; $&; "abcdefgh" =~ /d/
879
880perl would only copy the "abcd" part of the string. That could make a big
881difference in something like
882
883 $str = 'x' x 1_000_000;
884 $&; # whoops
885 $str =~ /x/g # one char copied a million times, not a million chars
886
887In Perl 5.20.0 a new copy-on-write system was enabled by default, which
888finally fixes all performance issues with these three variables, and makes
889them safe to use anywhere.
890
891The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
892find uses of these problematic match variables in your code.
13b0f67d 893
b0c22438 894=over 8
a0d0e21e 895
b0c22438 896=item $<I<digits>> ($1, $2, ...)
897X<$1> X<$2> X<$3>
8cc95fdb 898
b0c22438 899Contains the subpattern from the corresponding set of capturing
900parentheses from the last successful pattern match, not counting patterns
901matched in nested blocks that have been exited already.
8cc95fdb 902
b0c22438 903These variables are read-only and dynamically-scoped.
a043a685 904
b0c22438 905Mnemonic: like \digits.
a0d0e21e 906
b0c22438 907=item $MATCH
a0d0e21e 908
b0c22438 909=item $&
910X<$&> X<$MATCH>
a0d0e21e 911
b0c22438 912The string matched by the last successful pattern match (not counting
913any matches hidden within a BLOCK or C<eval()> enclosed by the current
914BLOCK).
a0d0e21e 915
40445027
DM
916See L</Performance issues> above for the serious performance implications
917of using this variable (even once) in your code.
80bca1b4 918
b0c22438 919This variable is read-only and dynamically-scoped.
f9cbb277 920
b0c22438 921Mnemonic: like C<&> in some editors.
0b9346e6 922
b0c22438 923=item ${^MATCH}
924X<${^MATCH}>
a0d0e21e 925
b0c22438 926This is similar to C<$&> (C<$MATCH>) except that it does not incur the
13b0f67d 927performance penalty associated with that variable.
40445027
DM
928
929See L</Performance issues> above.
930
13b0f67d 931In Perl v5.18 and earlier, it is only guaranteed
b0c22438 932to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
933the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
934C<${^MATCH}> does the same thing as C<$MATCH>.
80bca1b4 935
60cf4914 936This variable was added in Perl v5.10.0.
4bc88a62 937
b0c22438 938This variable is read-only and dynamically-scoped.
e2975953 939
b0c22438 940=item $PREMATCH
52c447a8 941
b0c22438 942=item $`
5b442a2a 943X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 944
b0c22438 945The string preceding whatever was matched by the last successful
946pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 947enclosed by the current BLOCK.
a0d0e21e 948
40445027
DM
949See L</Performance issues> above for the serious performance implications
950of using this variable (even once) in your code.
a0d0e21e 951
b0c22438 952This variable is read-only and dynamically-scoped.
a0d0e21e 953
b0c22438 954Mnemonic: C<`> often precedes a quoted string.
f83ed198 955
b0c22438 956=item ${^PREMATCH}
5b442a2a 957X<$`> X<${^PREMATCH}>
a0d0e21e 958
b0c22438 959This is similar to C<$`> ($PREMATCH) except that it does not incur the
13b0f67d 960performance penalty associated with that variable.
40445027
DM
961
962See L</Performance issues> above.
963
13b0f67d 964In Perl v5.18 and earlier, it is only guaranteed
b0c22438 965to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
966the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
967C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
a0d0e21e 968
4a70680a 969This variable was added in Perl v5.10.0.
a0d0e21e 970
b0c22438 971This variable is read-only and dynamically-scoped.
a0d0e21e 972
b0c22438 973=item $POSTMATCH
16070b82 974
b0c22438 975=item $'
5b442a2a 976X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 977
b0c22438 978The string following whatever was matched by the last successful
979pattern match (not counting any matches hidden within a BLOCK or C<eval()>
241a59d9 980enclosed by the current BLOCK). Example:
305aace0 981
9548c15c
FC
982 local $_ = 'abcdefghi';
983 /def/;
984 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 985
40445027
DM
986See L</Performance issues> above for the serious performance implications
987of using this variable (even once) in your code.
a0d0e21e 988
b0c22438 989This variable is read-only and dynamically-scoped.
990
991Mnemonic: C<'> often follows a quoted string.
992
993=item ${^POSTMATCH}
5b442a2a 994X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 995
996This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
13b0f67d 997performance penalty associated with that variable.
40445027
DM
998
999See L</Performance issues> above.
1000
13b0f67d 1001In Perl v5.18 and earlier, it is only guaranteed
b0c22438 1002to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
1003the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1004C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
b0c22438 1005
60cf4914 1006This variable was added in Perl v5.10.0.
b0c22438 1007
1008This variable is read-only and dynamically-scoped.
1009
1010=item $LAST_PAREN_MATCH
1011
1012=item $+
1013X<$+> X<$LAST_PAREN_MATCH>
1014
1015The text matched by the last bracket of the last successful search pattern.
1016This is useful if you don't know which one of a set of alternative patterns
241a59d9 1017matched. For example:
b0c22438 1018
9548c15c 1019 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1020
1021This variable is read-only and dynamically-scoped.
1022
1023Mnemonic: be positive and forward looking.
1024
1025=item $LAST_SUBMATCH_RESULT
1026
1027=item $^N
5b442a2a 1028X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1029
1030The text matched by the used group most-recently closed (i.e. the group
1031with the rightmost closing parenthesis) of the last successful search
1032pattern.
1033
1034This is primarily used inside C<(?{...})> blocks for examining text
241a59d9 1035recently matched. For example, to effectively capture text to a variable
b0c22438 1036(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1037
9548c15c 1038 (?:(...)(?{ $var = $^N }))
b0c22438 1039
1040By setting and then using C<$var> in this way relieves you from having to
1041worry about exactly which numbered set of parentheses they are.
1042
60cf4914 1043This variable was added in Perl v5.8.0.
b0c22438 1044
1045Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1046
1047=item @LAST_MATCH_END
1048
1049=item @+
1050X<@+> X<@LAST_MATCH_END>
1051
1052This array holds the offsets of the ends of the last successful
241a59d9
FC
1053submatches in the currently active dynamic scope. C<$+[0]> is
1054the offset into the string of the end of the entire match. This
b0c22438 1055is the same value as what the C<pos> function returns when called
241a59d9 1056on the variable that was matched against. The I<n>th element
b0c22438 1057of this array holds the offset of the I<n>th submatch, so
1058C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
241a59d9
FC
1059past where C<$2> ends, and so on. You can use C<$#+> to determine
1060how many subgroups were in the last successful match. See the
b0c22438 1061examples given for the C<@-> variable.
1062
60cf4914 1063This variable was added in Perl v5.6.0.
b0c22438 1064
1065=item %LAST_PAREN_MATCH
1066
1067=item %+
5b442a2a 1068X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1069
1070Similar to C<@+>, the C<%+> hash allows access to the named capture
1071buffers, should they exist, in the last successful match in the
1072currently active dynamic scope.
1073
1074For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1075
9548c15c 1076 'foo' =~ /(?<foo>foo)/;
b0c22438 1077
1078The keys of the C<%+> hash list only the names of buffers that have
1079captured (and that are thus associated to defined values).
1080
1081The underlying behaviour of C<%+> is provided by the
1082L<Tie::Hash::NamedCapture> module.
1083
1084B<Note:> C<%-> and C<%+> are tied views into a common internal hash
241a59d9 1085associated with the last successful regular expression. Therefore mixing
b0c22438 1086iterative access to them via C<each> may have unpredictable results.
1087Likewise, if the last successful match changes, then the results may be
1088surprising.
1089
60cf4914 1090This variable was added in Perl v5.10.0.
a0d0e21e 1091
b0c22438 1092This variable is read-only and dynamically-scoped.
1093
1094=item @LAST_MATCH_START
1095
1096=item @-
1097X<@-> X<@LAST_MATCH_START>
1098
1099C<$-[0]> is the offset of the start of the last successful match.
1100C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1101I<n>-th subpattern, or undef if the subpattern did not match.
1102
1103Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
241a59d9 1104$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
b0c22438 1105$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
241a59d9
FC
1106C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1107last matched subgroup in the last successful match. Contrast with
1108C<$#+>, the number of subgroups in the regular expression. Compare
b0c22438 1109with C<@+>.
1110
1111This array holds the offsets of the beginnings of the last
1112successful submatches in the currently active dynamic scope.
1113C<$-[0]> is the offset into the string of the beginning of the
241a59d9 1114entire match. The I<n>th element of this array holds the offset
b0c22438 1115of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1116begins, C<$-[2]> the offset where C<$2> begins, and so on.
1117
1118After a match against some variable C<$var>:
1119
1120=over 5
1121
1122=item C<$`> is the same as C<substr($var, 0, $-[0])>
1123
1124=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1125
1126=item C<$'> is the same as C<substr($var, $+[0])>
1127
1128=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1129
1130=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1131
1132=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1133
1134=back
1135
60cf4914 1136This variable was added in Perl v5.6.0.
b0c22438 1137
5b442a2a 1138=item %LAST_MATCH_START
1139
b0c22438 1140=item %-
5b442a2a 1141X<%-> X<%LAST_MATCH_START>
b0c22438 1142
1143Similar to C<%+>, this variable allows access to the named capture groups
241a59d9 1144in the last successful match in the currently active dynamic scope. To
b0c22438 1145each capture group name found in the regular expression, it associates a
1146reference to an array containing the list of values captured by all
1147buffers with that name (should there be several of them), in the order
1148where they appear.
1149
1150Here's an example:
1151
1152 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1153 foreach my $bufname (sort keys %-) {
1154 my $ary = $-{$bufname};
1155 foreach my $idx (0..$#$ary) {
1156 print "\$-{$bufname}[$idx] : ",
9548c15c
FC
1157 (defined($ary->[$idx])
1158 ? "'$ary->[$idx]'"
1159 : "undef"),
b0c22438 1160 "\n";
1161 }
1162 }
1163 }
1164
1165would print out:
1166
9548c15c
FC
1167 $-{A}[0] : '1'
1168 $-{A}[1] : '3'
1169 $-{B}[0] : '2'
1170 $-{B}[1] : '4'
b0c22438 1171
1172The keys of the C<%-> hash correspond to all buffer names found in
1173the regular expression.
1174
1175The behaviour of C<%-> is implemented via the
1176L<Tie::Hash::NamedCapture> module.
1177
1178B<Note:> C<%-> and C<%+> are tied views into a common internal hash
241a59d9 1179associated with the last successful regular expression. Therefore mixing
b0c22438 1180iterative access to them via C<each> may have unpredictable results.
1181Likewise, if the last successful match changes, then the results may be
1182surprising.
1183
60cf4914 1184This variable was added in Perl v5.10.0.
b0c22438 1185
1186This variable is read-only and dynamically-scoped.
1187
1188=item $LAST_REGEXP_CODE_RESULT
1189
1190=item $^R
1191X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1192
1193The result of evaluation of the last successful C<(?{ code })>
241a59d9 1194regular expression assertion (see L<perlre>). May be written to.
b0c22438 1195
1196This variable was added in Perl 5.005.
a0d0e21e 1197
a3621e74 1198=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1199X<${^RE_DEBUG_FLAGS}>
a3621e74 1200
241a59d9
FC
1201The current value of the regex debugging flags. Set to 0 for no debug output
1202even when the C<re 'debug'> module is loaded. See L<re> for details.
b0c22438 1203
60cf4914 1204This variable was added in Perl v5.10.0.
a3621e74 1205
0111c4fd 1206=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1207X<${^RE_TRIE_MAXBUF}>
a3621e74
YO
1208
1209Controls how certain regex optimisations are applied and how much memory they
241a59d9
FC
1210utilize. This value by default is 65536 which corresponds to a 512kB
1211temporary cache. Set this to a higher value to trade
1212memory for speed when matching large alternations. Set
1213it to a lower value if you want the optimisations to
a3621e74
YO
1214be as conservative of memory as possible but still occur, and set it to a
1215negative value to prevent the optimisation and conserve the most memory.
1216Under normal situations this variable should be of no interest to you.
1217
60cf4914 1218This variable was added in Perl v5.10.0.
a0d0e21e 1219
b0c22438 1220=back
a0d0e21e 1221
b0c22438 1222=head2 Variables related to filehandles
a0d0e21e 1223
b0c22438 1224Variables that depend on the currently selected filehandle may be set
1225by calling an appropriate object method on the C<IO::Handle> object,
1226although this is less efficient than using the regular built-in
241a59d9 1227variables. (Summary lines below for this contain the word HANDLE.)
b0c22438 1228First you must say
6e2995f4 1229
9548c15c 1230 use IO::Handle;
0462a1ab 1231
b0c22438 1232after which you may use either
0462a1ab 1233
9548c15c 1234 method HANDLE EXPR
0462a1ab 1235
b0c22438 1236or more safely,
0462a1ab 1237
9548c15c 1238 HANDLE->method(EXPR)
0462a1ab 1239
241a59d9 1240Each method returns the old value of the C<IO::Handle> attribute. The
b0c22438 1241methods each take an optional EXPR, which, if supplied, specifies the
241a59d9 1242new value for the C<IO::Handle> attribute in question. If not
b0c22438 1243supplied, most methods do nothing to the current value--except for
1244C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1245
b0c22438 1246Because loading in the C<IO::Handle> class is an expensive operation,
1247you should learn how to use the regular built-in variables.
1248
241a59d9 1249A few of these variables are considered "read-only". This means that
b0c22438 1250if you try to assign to this variable, either directly or indirectly
1251through a reference, you'll raise a run-time exception.
1252
1253You should be very careful when modifying the default values of most
241a59d9 1254special variables described in this document. In most cases you want
b0c22438 1255to localize these variables before changing them, since if you don't,
1256the change may affect other modules which rely on the default values
241a59d9 1257of the special variables that you have changed. This is one of the
b0c22438 1258correct ways to read the whole file at once:
1259
9548c15c
FC
1260 open my $fh, "<", "foo" or die $!;
1261 local $/; # enable localized slurp mode
1262 my $content = <$fh>;
1263 close $fh;
b0c22438 1264
1265But the following code is quite bad:
1266
9548c15c
FC
1267 open my $fh, "<", "foo" or die $!;
1268 undef $/; # enable slurp mode
1269 my $content = <$fh>;
1270 close $fh;
b0c22438 1271
1272since some other module, may want to read data from some file in the
1273default "line mode", so if the code we have just presented has been
1274executed, the global value of C<$/> is now changed for any other code
1275running inside the same Perl interpreter.
1276
1277Usually when a variable is localized you want to make sure that this
241a59d9
FC
1278change affects the shortest scope possible. So unless you are already
1279inside some short C<{}> block, you should create one yourself. For
b0c22438 1280example:
1281
9548c15c
FC
1282 my $content = '';
1283 open my $fh, "<", "foo" or die $!;
1284 {
1285 local $/;
1286 $content = <$fh>;
1287 }
1288 close $fh;
0462a1ab 1289
b0c22438 1290Here is an example of how your own code can go broken:
0462a1ab 1291
9548c15c
FC
1292 for ( 1..3 ){
1293 $\ = "\r\n";
1294 nasty_break();
1295 print "$_";
1296 }
0b9346e6 1297
9548c15c 1298 sub nasty_break {
0b9346e6 1299 $\ = "\f";
1300 # do something with $_
9548c15c 1301 }
0462a1ab 1302
0b9346e6 1303You probably expect this code to print the equivalent of
0462a1ab 1304
0b9346e6 1305 "1\r\n2\r\n3\r\n"
0462a1ab 1306
b0c22438 1307but instead you get:
0462a1ab 1308
0b9346e6 1309 "1\f2\f3\f"
0462a1ab 1310
0b9346e6 1311Why? Because C<nasty_break()> modifies C<$\> without localizing it
241a59d9
FC
1312first. The value you set in C<nasty_break()> is still there when you
1313return. The fix is to add C<local()> so the value doesn't leak out of
0b9346e6 1314C<nasty_break()>:
6e2995f4 1315
9548c15c 1316 local $\ = "\f";
a0d0e21e 1317
b0c22438 1318It's easy to notice the problem in such a short example, but in more
1319complicated code you are looking for trouble if you don't localize
1320changes to the special variables.
a0d0e21e 1321
b0c22438 1322=over 8
a0d0e21e 1323
b0c22438 1324=item $ARGV
1325X<$ARGV>
fb73857a 1326
ca1b95ae 1327Contains the name of the current file when reading from C<< <> >>.
b0c22438 1328
1329=item @ARGV
1330X<@ARGV>
1331
ca1b95ae 1332The array C<@ARGV> contains the command-line arguments intended for
241a59d9 1333the script. C<$#ARGV> is generally the number of arguments minus
b0c22438 1334one, because C<$ARGV[0]> is the first argument, I<not> the program's
241a59d9 1335command name itself. See L</$0> for the command name.
b0c22438 1336
84dabc03 1337=item ARGV
1338X<ARGV>
1339
1340The special filehandle that iterates over command-line filenames in
241a59d9
FC
1341C<@ARGV>. Usually written as the null filehandle in the angle operator
1342C<< <> >>. Note that currently C<ARGV> only has its magical effect
84dabc03 1343within the C<< <> >> operator; elsewhere it is just a plain filehandle
241a59d9 1344corresponding to the last file opened by C<< <> >>. In particular,
84dabc03 1345passing C<\*ARGV> as a parameter to a function that expects a filehandle
1346may not cause your function to automatically read the contents of all the
1347files in C<@ARGV>.
1348
b0c22438 1349=item ARGVOUT
1350X<ARGVOUT>
1351
1352The special filehandle that points to the currently open output file
241a59d9
FC
1353when doing edit-in-place processing with B<-i>. Useful when you have
1354to do a lot of inserting and don't want to keep modifying C<$_>. See
b0c22438 1355L<perlrun> for the B<-i> switch.
1356
96948506 1357=item IO::Handle->output_field_separator( EXPR )
84dabc03 1358
1359=item $OUTPUT_FIELD_SEPARATOR
1360
1361=item $OFS
1362
1363=item $,
1364X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1365
241a59d9
FC
1366The output field separator for the print operator. If defined, this
1367value is printed between each of print's arguments. Default is C<undef>.
84dabc03 1368
96948506 1369You cannot call C<output_field_separator()> on a handle, only as a
008f9687 1370static method. See L<IO::Handle|IO::Handle>.
96948506 1371
84dabc03 1372Mnemonic: what is printed when there is a "," in your print statement.
1373
5b442a2a 1374=item HANDLE->input_line_number( EXPR )
b0c22438 1375
1376=item $INPUT_LINE_NUMBER
1377
1378=item $NR
1379
1380=item $.
1381X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1382
1383Current line number for the last filehandle accessed.
1384
1385Each filehandle in Perl counts the number of lines that have been read
241a59d9 1386from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1387constitutes a line may not match yours.) When a line is read from a
1388filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1389C<seek()> is called on it, C<$.> becomes an alias to the line counter
1390for that filehandle.
1391
1392You can adjust the counter by assigning to C<$.>, but this will not
241a59d9
FC
1393actually move the seek pointer. I<Localizing C<$.> will not localize
1394the filehandle's line count>. Instead, it will localize perl's notion
b0c22438 1395of which filehandle C<$.> is currently aliased to.
1396
1397C<$.> is reset when the filehandle is closed, but B<not> when an open
241a59d9
FC
1398filehandle is reopened without an intervening C<close()>. For more
1399details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
b0c22438 1400an explicit close, line numbers increase across C<ARGV> files (but see
1401examples in L<perlfunc/eof>).
1402
1403You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1404line counter for a given filehandle without having to worry about
1405which handle you last accessed.
1406
1407Mnemonic: many programs use "." to mean the current line number.
1408
96948506 1409=item IO::Handle->input_record_separator( EXPR )
b0c22438 1410
1411=item $INPUT_RECORD_SEPARATOR
1412
1413=item $RS
1414
1415=item $/
1416X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1417
241a59d9
FC
1418The input record separator, newline by default. This influences Perl's
1419idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1420treating empty lines as a terminator if set to the null string (an
241a59d9 1421empty line cannot contain any spaces or tabs). You may set it to a
84dabc03 1422multi-character string to match a multi-character terminator, or to
241a59d9 1423C<undef> to read through the end of file. Setting it to C<"\n\n">
84dabc03 1424means something slightly different than setting to C<"">, if the file
241a59d9
FC
1425contains consecutive empty lines. Setting to C<""> will treat two or
1426more consecutive empty lines as a single empty line. Setting to
84dabc03 1427C<"\n\n"> will blindly assume that the next input character belongs to
1428the next paragraph, even if it's a newline.
b0c22438 1429
1430 local $/; # enable "slurp" mode
1431 local $_ = <FH>; # whole file now here
1432 s/\n[ \t]+/ /g;
1433
241a59d9 1434Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1435be better for something. :-)
1436
1437Setting C<$/> to a reference to an integer, scalar containing an
1438integer, or scalar that's convertible to an integer will attempt to
1439read records instead of lines, with the maximum record size being the
3d249121 1440referenced integer number of characters. So this:
b0c22438 1441
1442 local $/ = \32768; # or \"32768", or \$var_containing_32768
1443 open my $fh, "<", $myfile or die $!;
1444 local $_ = <$fh>;
fb73857a 1445
f1ee460b 1446will read a record of no more than 32768 characters from $fh. If you're
b0c22438 1447not reading from a record-oriented file (or your OS doesn't have
1448record-oriented files), then you'll likely get a full chunk of data
241a59d9
FC
1449with every read. If a record is larger than the record size you've
1450set, you'll get the record back in pieces. Trying to set the record
b3a2acfa
YO
1451size to zero or less is deprecated and will cause $/ to have the value
1452of "undef", which will cause reading in the (rest of the) whole file.
1453
1454As of 5.19.9 setting C<$/> to any other form of reference will throw a
1455fatal exception. This is in preparation for supporting new ways to set
1456C<$/> in the future.
6e2995f4 1457
78c28381 1458On VMS only, record reads bypass PerlIO layers and any associated
3d249121 1459buffering, so you must not mix record and non-record reads on the
78c28381
CB
1460same filehandle. Record mode mixes with line mode only when the
1461same buffering layer is in use for both modes.
5c055ba3 1462
96948506 1463You cannot call C<input_record_separator()> on a handle, only as a
008f9687 1464static method. See L<IO::Handle|IO::Handle>.
96948506 1465
008f9687 1466See also L<perlport/"Newlines">. Also see L</$.>.
9bf22702 1467
b0c22438 1468Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1469
96948506 1470=item IO::Handle->output_record_separator( EXPR )
84902520 1471
b0c22438 1472=item $OUTPUT_RECORD_SEPARATOR
84902520 1473
b0c22438 1474=item $ORS
84902520 1475
b0c22438 1476=item $\
1477X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
84902520 1478
241a59d9
FC
1479The output record separator for the print operator. If defined, this
1480value is printed after the last of print's arguments. Default is C<undef>.
84902520 1481
96948506 1482You cannot call C<output_record_separator()> on a handle, only as a
008f9687 1483static method. See L<IO::Handle|IO::Handle>.
96948506 1484
b0c22438 1485Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1486Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1487
5b442a2a 1488=item HANDLE->autoflush( EXPR )
1489
1490=item $OUTPUT_AUTOFLUSH
1491
84dabc03 1492=item $|
1493X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1494
84dabc03 1495If set to nonzero, forces a flush right away and after every write or
241a59d9 1496print on the currently selected output channel. Default is 0
84dabc03 1497(regardless of whether the channel is really buffered by the system or
1498not; C<$|> tells you only whether you've asked Perl explicitly to
241a59d9
FC
1499flush after each write). STDOUT will typically be line buffered if
1500output is to the terminal and block buffered otherwise. Setting this
84dabc03 1501variable is useful primarily when you are outputting to a pipe or
1502socket, such as when you are running a Perl program under B<rsh> and
241a59d9
FC
1503want to see the output as it's happening. This has no effect on input
1504buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1505how to select the output channel. See also L<IO::Handle>.
84dabc03 1506
1507Mnemonic: when you want your pipes to be piping hot.
1508
8561ea1d
FC
1509=item ${^LAST_FH}
1510X<${^LAST_FH}>
1511
1512This read-only variable contains a reference to the last-read filehandle.
1513This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1514This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1515use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1516an error or warning message.
1517
1518This variable was added in Perl v5.18.0.
1519
84dabc03 1520=back
84902520 1521
b0c22438 1522=head3 Variables related to formats
83ee9e09 1523
b0c22438 1524The special variables for formats are a subset of those for
241a59d9 1525filehandles. See L<perlform> for more information about Perl's
69b55ccc 1526formats.
83ee9e09 1527
b0c22438 1528=over 8
83ee9e09 1529
84dabc03 1530=item $ACCUMULATOR
1531
1532=item $^A
1533X<$^A> X<$ACCUMULATOR>
1534
1535The current value of the C<write()> accumulator for C<format()> lines.
1536A format contains C<formline()> calls that put their result into
241a59d9
FC
1537C<$^A>. After calling its format, C<write()> prints out the contents
1538of C<$^A> and empties. So you never really see the contents of C<$^A>
1539unless you call C<formline()> yourself and then look at it. See
96090e4f 1540L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
84dabc03 1541
96948506 1542=item IO::Handle->format_formfeed(EXPR)
5b442a2a 1543
1544=item $FORMAT_FORMFEED
1545
84dabc03 1546=item $^L
1547X<$^L> X<$FORMAT_FORMFEED>
1548
241a59d9 1549What formats output as a form feed. The default is C<\f>.
84dabc03 1550
96948506 1551You cannot call C<format_formfeed()> on a handle, only as a static
008f9687 1552method. See L<IO::Handle|IO::Handle>.
96948506 1553
b0c22438 1554=item HANDLE->format_page_number(EXPR)
83ee9e09 1555
b0c22438 1556=item $FORMAT_PAGE_NUMBER
83ee9e09 1557
b0c22438 1558=item $%
1559X<$%> X<$FORMAT_PAGE_NUMBER>
83ee9e09 1560
b0c22438 1561The current page number of the currently selected output channel.
83ee9e09 1562
b0c22438 1563Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1564
b0c22438 1565=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1566
b0c22438 1567=item $FORMAT_LINES_LEFT
66558a10 1568
b0c22438 1569=item $-
1570X<$-> X<$FORMAT_LINES_LEFT>
fb73857a 1571
b0c22438 1572The number of lines left on the page of the currently selected output
1573channel.
fa05a9fd 1574
b0c22438 1575Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1576
96948506 1577=item IO::Handle->format_line_break_characters EXPR
fb73857a 1578
84dabc03 1579=item $FORMAT_LINE_BREAK_CHARACTERS
a0d0e21e 1580
84dabc03 1581=item $:
1582X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e 1583
84dabc03 1584The current set of characters after which a string may be broken to
241a59d9 1585fill continuation fields (starting with C<^>) in a format. The default is
84dabc03 1586S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1587
96948506 1588You cannot call C<format_line_break_characters()> on a handle, only as
008f9687 1589a static method. See L<IO::Handle|IO::Handle>.
96948506 1590
84dabc03 1591Mnemonic: a "colon" in poetry is a part of a line.
1592
1593=item HANDLE->format_lines_per_page(EXPR)
1594
1595=item $FORMAT_LINES_PER_PAGE
1596
1597=item $=
1598X<$=> X<$FORMAT_LINES_PER_PAGE>
1599
1600The current page length (printable lines) of the currently selected
241a59d9 1601output channel. The default is 60.
84dabc03 1602
1603Mnemonic: = has horizontal lines.
7c36658b 1604
b0c22438 1605=item HANDLE->format_top_name(EXPR)
7c36658b 1606
b0c22438 1607=item $FORMAT_TOP_NAME
a05d7ebb 1608
b0c22438 1609=item $^
1610X<$^> X<$FORMAT_TOP_NAME>
fde18df1 1611
b0c22438 1612The name of the current top-of-page format for the currently selected
241a59d9
FC
1613output channel. The default is the name of the filehandle with C<_TOP>
1614appended. For example, the default format top name for the C<STDOUT>
12abbafd 1615filehandle is C<STDOUT_TOP>.
e07ea26a 1616
b0c22438 1617Mnemonic: points to top of page.
e07ea26a 1618
84dabc03 1619=item HANDLE->format_name(EXPR)
16070b82 1620
84dabc03 1621=item $FORMAT_NAME
aa2f2a36 1622
84dabc03 1623=item $~
1624X<$~> X<$FORMAT_NAME>
aa2f2a36 1625
84dabc03 1626The name of the current report format for the currently selected
241a59d9
FC
1627output channel. The default format name is the same as the filehandle
1628name. For example, the default format name for the C<STDOUT>
84dabc03 1629filehandle is just C<STDOUT>.
16070b82 1630
84dabc03 1631Mnemonic: brother to C<$^>.
16070b82 1632
b0c22438 1633=back
a0d0e21e 1634
84dabc03 1635=head2 Error Variables
b0c22438 1636X<error> X<exception>
a0d0e21e 1637
b0c22438 1638The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1639about different types of error conditions that may appear during
241a59d9 1640execution of a Perl program. The variables are shown ordered by
b0c22438 1641the "distance" between the subsystem which reported the error and
241a59d9 1642the Perl process. They correspond to errors detected by the Perl
b0c22438 1643interpreter, C library, operating system, or an external program,
1644respectively.
4438c4b7 1645
b0c22438 1646To illustrate the differences between these variables, consider the
241a59d9 1647following Perl expression, which uses a single-quoted string. After
7fd683ff 1648execution of this statement, perl may have set all four special error
7333b1c4 1649variables:
4438c4b7 1650
9548c15c
FC
1651 eval q{
1652 open my $pipe, "/cdrom/install |" or die $!;
1653 my @res = <$pipe>;
1654 close $pipe or die "bad pipe: $?, $!";
1655 };
a0d0e21e 1656
7333b1c4 1657When perl executes the C<eval()> expression, it translates the
1658C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
241a59d9 1659and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1660the C library's C<errno> if one of these calls fails.
2a8c8378 1661
84dabc03 1662C<$@> is set if the string to be C<eval>-ed did not compile (this may
1663happen if C<open> or C<close> were imported with bad prototypes), or
241a59d9 1664if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1665value of C<$@> is the compile error, or the argument to C<die> (which
241a59d9 1666will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1667
84dabc03 1668Under a few operating systems, C<$^E> may contain a more verbose error
241a59d9 1669indicator, such as in this case, "CDROM tray not closed." Systems that
84dabc03 1670do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1671
b0c22438 1672Finally, C<$?> may be set to non-0 value if the external program
241a59d9 1673F</cdrom/install> fails. The upper eight bits reflect specific error
84dabc03 1674conditions encountered by the program (the program's C<exit()> value).
1675The lower eight bits reflect mode of failure, like signal death and
241a59d9 1676core dump information. See L<wait(2)> for details. In contrast to
84dabc03 1677C<$!> and C<$^E>, which are set only if error condition is detected,
1678the variable C<$?> is set on each C<wait> or pipe C<close>,
241a59d9 1679overwriting the old value. This is more like C<$@>, which on every
84dabc03 1680C<eval()> is always set on failure and cleared on success.
a0d0e21e 1681
b0c22438 1682For more details, see the individual descriptions at C<$@>, C<$!>,
1683C<$^E>, and C<$?>.
38e4f4ae 1684
0b9346e6 1685=over 8
1686
b0c22438 1687=item ${^CHILD_ERROR_NATIVE}
1688X<$^CHILD_ERROR_NATIVE>
a0d0e21e 1689
b0c22438 1690The native status returned by the last pipe close, backtick (C<``>)
1691command, successful call to C<wait()> or C<waitpid()>, or from the
241a59d9 1692C<system()> operator. On POSIX-like systems this value can be decoded
b0c22438 1693with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1694WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1695
b0c22438 1696Under VMS this reflects the actual VMS exit status; i.e. it is the
1697same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1698
60cf4914 1699This variable was added in Perl v5.10.0.
a0d0e21e 1700
5b442a2a 1701=item $EXTENDED_OS_ERROR
1702
84dabc03 1703=item $^E
1704X<$^E> X<$EXTENDED_OS_ERROR>
1705
241a59d9 1706Error information specific to the current operating system. At the
84dabc03 1707moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
241a59d9 1708for MacPerl). On all other platforms, C<$^E> is always just the same
84dabc03 1709as C<$!>.
1710
1711Under VMS, C<$^E> provides the VMS status value from the last system
241a59d9
FC
1712error. This is more specific information about the last system error
1713than that provided by C<$!>. This is particularly important when C<$!>
84dabc03 1714is set to B<EVMSERR>.
1715
1716Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1717API either via CRT, or directly from perl.
1718
1719Under Win32, C<$^E> always returns the last error information reported
1720by the Win32 call C<GetLastError()> which describes the last error
241a59d9
FC
1721from within the Win32 API. Most Win32-specific code will report errors
1722via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
84dabc03 1723portable Perl code will report errors via C<$!>.
1724
1725Caveats mentioned in the description of C<$!> generally apply to
1726C<$^E>, also.
1727
1728This variable was added in Perl 5.003.
1729
1730Mnemonic: Extra error explanation.
0b9346e6 1731
84dabc03 1732=item $EXCEPTIONS_BEING_CAUGHT
1733
1734=item $^S
1735X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1736
1737Current state of the interpreter.
1738
ca1b95ae 1739 $^S State
aa959a20
FC
1740 --------- -------------------------------------
1741 undef Parsing module, eval, or main program
ca1b95ae 1742 true (1) Executing an eval
1743 false (0) Otherwise
84dabc03 1744
1745The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1746handlers.
1747
aa959a20
FC
1748The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1749the C<undef> value does not indicate whether exceptions are being caught,
1750since compilation of the main program does not catch exceptions.
1751
84dabc03 1752This variable was added in Perl 5.004.
1753
1754=item $WARNING
1755
1756=item $^W
1757X<$^W> X<$WARNING>
1758
1759The current value of the warning switch, initially true if B<-w> was
1760used, false otherwise, but directly modifiable.
1761
1762See also L<warnings>.
1763
0b9346e6 1764Mnemonic: related to the B<-w> switch.
84dabc03 1765
1766=item ${^WARNING_BITS}
ca1b95ae 1767X<${^WARNING_BITS}>
84dabc03 1768
1769The current set of warning checks enabled by the C<use warnings> pragma.
44567c86
FC
1770It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1771values are considered internal to the L<warnings> pragma and may change
1772between versions of Perl.
84dabc03 1773
60cf4914 1774This variable was added in Perl v5.6.0.
84dabc03 1775
b0c22438 1776=item $OS_ERROR
5ccee41e 1777
b0c22438 1778=item $ERRNO
5ccee41e 1779
b0c22438 1780=item $!
1781X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1782
a73bef78
JL
1783When referenced, C<$!> retrieves the current value
1784of the C C<errno> integer variable.
1785If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1786When referenced as a string, C<$!> yields the system error string
1787corresponding to C<errno>.
1788
1789Many system or library calls set C<errno> if they fail,
1790to indicate the cause of failure. They usually do B<not>
1791set C<errno> to zero if they succeed. This means C<errno>,
1792hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1793
1794 if (open my $fh, "<", $filename) {
ca1b95ae 1795 # Here $! is meaningless.
1796 ...
7fd683ff 1797 }
ca1b95ae 1798 else {
1799 # ONLY here is $! meaningful.
1800 ...
1801 # Already here $! might be meaningless.
b0c22438 1802 }
1803 # Since here we might have either success or failure,
a73bef78 1804 # $! is meaningless.
a0d0e21e 1805
a73bef78
JL
1806Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1807of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1808It can be used immediately before invoking the C<die()> operator,
1809to set the exit value, or to inspect the system error string
1810corresponding to error I<n>, or to restore C<$!> to a meaningful state.
d54b56d5 1811
b0c22438 1812Mnemonic: What just went bang?
314d39ce 1813
b0c22438 1814=item %OS_ERROR
fb73857a 1815
b0c22438 1816=item %ERRNO
fb73857a 1817
b0c22438 1818=item %!
5b442a2a 1819X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1820
b0c22438 1821Each element of C<%!> has a true value only if C<$!> is set to that
241a59d9 1822value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1823value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1824such file or directory" (or its moral equivalent: not all operating
241a59d9 1825systems give that exact error, and certainly not all languages). To
84dabc03 1826check if a particular key is meaningful on your system, use C<exists
241a59d9 1827$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
7333b1c4 1828for more information, and also see L</$!>.
a0d0e21e 1829
b0c22438 1830This variable was added in Perl 5.005.
44f0be63 1831
84dabc03 1832=item $CHILD_ERROR
b687b08b 1833
84dabc03 1834=item $?
1835X<$?> X<$CHILD_ERROR>
a0d0e21e 1836
84dabc03 1837The status returned by the last pipe close, backtick (C<``>) command,
1838successful call to C<wait()> or C<waitpid()>, or from the C<system()>
241a59d9 1839operator. This is just the 16-bit status word returned by the
84dabc03 1840traditional Unix C<wait()> system call (or else is made up to look
241a59d9 1841like it). Thus, the exit value of the subprocess is really (C<<< $? >>
84dabc03 18428 >>>), and C<$? & 127> gives which signal, if any, the process died
1843from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1844
84dabc03 1845Additionally, if the C<h_errno> variable is supported in C, its value
1846is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1847
84dabc03 1848If you have installed a signal handler for C<SIGCHLD>, the
1849value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1850
84dabc03 1851Inside an C<END> subroutine C<$?> contains the value that is going to be
241a59d9
FC
1852given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1853change the exit status of your program. For example:
a0d0e21e 1854
84dabc03 1855 END {
1856 $? = 1 if $? == 255; # die would make it 255
1857 }
a0d0e21e 1858
84dabc03 1859Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1860actual VMS exit status, instead of the default emulation of POSIX
1861status; see L<perlvms/$?> for details.
1862
1863Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1864
b0c22438 1865=item $EVAL_ERROR
f648820c 1866
b0c22438 1867=item $@
1868X<$@> X<$EVAL_ERROR>
a0d0e21e 1869
241a59d9
FC
1870The Perl syntax error message from the
1871last C<eval()> operator. If C<$@> is
0b9346e6 1872the null string, the last C<eval()> parsed and executed correctly
b0c22438 1873(although the operations you invoked may have failed in the normal
1874fashion).
a0d0e21e 1875
241a59d9 1876Warning messages are not collected in this variable. You can, however,
b0c22438 1877set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 1878described in L</%SIG>.
748a9306 1879
b0c22438 1880Mnemonic: Where was the syntax error "at"?
7f315d2e 1881
b0c22438 1882=back
7f315d2e 1883
1fa81471
DR
1884=head2 Variables related to the interpreter state
1885
1886These variables provide information about the current interpreter state.
1887
1888=over 8
1889
1890=item $COMPILING
1891
1892=item $^C
1893X<$^C> X<$COMPILING>
1894
1895The current value of the flag associated with the B<-c> switch.
1896Mainly of use with B<-MO=...> to allow code to alter its behavior
1897when being compiled, such as for example to C<AUTOLOAD> at compile
241a59d9 1898time rather than normal, deferred loading. Setting
1fa81471
DR
1899C<$^C = 1> is similar to calling C<B::minus_c>.
1900
60cf4914 1901This variable was added in Perl v5.6.0.
1fa81471
DR
1902
1903=item $DEBUGGING
1904
1905=item $^D
1906X<$^D> X<$DEBUGGING>
1907
241a59d9 1908The current value of the debugging flags. May be read or set. Like its
1fa81471
DR
1909command-line equivalent, you can use numeric or symbolic values, eg
1910C<$^D = 10> or C<$^D = "st">.
1911
1912Mnemonic: value of B<-D> switch.
1913
1914=item ${^ENCODING}
1915X<${^ENCODING}>
1916
a3ee04ba
KW
1917DEPRECATED!!!
1918
1fa81471 1919The I<object reference> to the C<Encode> object that is used to convert
241a59d9 1920the source code to Unicode. Thanks to this variable your Perl script
a3ee04ba
KW
1921does not have to be written in UTF-8. Default is C<undef>.
1922
1923Setting this variable to any other value than C<undef> is deprecated due
1924to fundamental defects in its design and implementation. It is planned
1925to remove it from a future Perl version. Its purpose was to allow your
1926non-ASCII Perl scripts to not have to be written in UTF-8; this was
1927useful before editors that worked on UTF-8 encoded text were common, but
1928that was long ago. It causes problems, such as affecting the operation
1929of other modules that aren't expecting it, causing general mayhem. Its
1930use can lead to segfaults.
1931
1932If you need something like this functionality, you should use the
1933L<encoding> pragma, which is also deprecated, but has fewer nasty side
1934effects.
1935
1936If you are coming here because code of yours is being adversely affected
1937by someone's use of this variable, you can usually work around it by
1938doing this:
1939
1940 local ${^ENCODING};
1941
1942near the beginning of the functions that are getting broken. This
1943undefines the variable during the scope of execution of the including
1944function.
1fa81471
DR
1945
1946This variable was added in Perl 5.8.2.
1947
1948=item ${^GLOBAL_PHASE}
1949X<${^GLOBAL_PHASE}>
1950
1951The current phase of the perl interpreter.
1952
1953Possible values are:
1954
1955=over 8
1956
1957=item CONSTRUCT
1958
241a59d9 1959The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1fa81471 1960value is mostly there for completeness and for use via the
241a59d9 1961underlying C variable C<PL_phase>. It's not really possible for Perl
1fa81471
DR
1962code to be executed unless construction of the interpreter is
1963finished.
1964
1965=item START
1966
241a59d9 1967This is the global compile-time. That includes, basically, every
1fa81471
DR
1968C<BEGIN> block executed directly or indirectly from during the
1969compile-time of the top-level program.
1970
1971This phase is not called "BEGIN" to avoid confusion with
1972C<BEGIN>-blocks, as those are executed during compile-time of any
241a59d9 1973compilation unit, not just the top-level program. A new, localised
1fa81471
DR
1974compile-time entered at run-time, for example by constructs as
1975C<eval "use SomeModule"> are not global interpreter phases, and
1976therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1977
1978=item CHECK
1979
1980Execution of any C<CHECK> blocks.
1981
1982=item INIT
1983
1984Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1985
1986=item RUN
1987
1988The main run-time, i.e. the execution of C<PL_main_root>.
1989
1990=item END
1991
1992Execution of any C<END> blocks.
1993
1994=item DESTRUCT
1995
1996Global destruction.
1997
1998=back
1999
241a59d9 2000Also note that there's no value for UNITCHECK-blocks. That's because
1fa81471
DR
2001those are run for each compilation unit individually, and therefore is
2002not a global interpreter phase.
2003
2004Not every program has to go through each of the possible phases, but
2005transition from one phase to another can only happen in the order
2006described in the above list.
2007
2008An example of all of the phases Perl code can see:
2009
2010 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
2011
2012 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
2013
2014 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
2015
2016 {
2017 package Print::Phase;
2018
2019 sub new {
2020 my ($class, $time) = @_;
2021 return bless \$time, $class;
2022 }
2023
2024 sub DESTROY {
2025 my $self = shift;
2026 print "$$self: ${^GLOBAL_PHASE}\n";
2027 }
2028 }
2029
2030 print "run-time: ${^GLOBAL_PHASE}\n";
2031
2032 my $runtime = Print::Phase->new(
2033 "lexical variables are garbage collected before END"
2034 );
2035
2036 END { print "end-time: ${^GLOBAL_PHASE}\n" }
2037
2038 our $destruct = Print::Phase->new(
2039 "package variables are garbage collected after END"
2040 );
2041
2042This will print out
2043
2044 compile-time: START
2045 check-time: CHECK
2046 init-time: INIT
2047 run-time: RUN
2048 lexical variables are garbage collected before END: RUN
2049 end-time: END
2050 package variables are garbage collected after END: DESTRUCT
2051
2052This variable was added in Perl 5.14.0.
2053
2054=item $^H
2055X<$^H>
2056
241a59d9
FC
2057WARNING: This variable is strictly for
2058internal use only. Its availability,
1fa81471
DR
2059behavior, and contents are subject to change without notice.
2060
241a59d9 2061This variable contains compile-time hints for the Perl interpreter. At the
1fa81471
DR
2062end of compilation of a BLOCK the value of this variable is restored to the
2063value when the interpreter started to compile the BLOCK.
2064
2065When perl begins to parse any block construct that provides a lexical scope
2066(e.g., eval body, required file, subroutine body, loop body, or conditional
2067block), the existing value of C<$^H> is saved, but its value is left unchanged.
2068When the compilation of the block is completed, it regains the saved value.
2069Between the points where its value is saved and restored, code that
2070executes within BEGIN blocks is free to change the value of C<$^H>.
2071
2072This behavior provides the semantic of lexical scoping, and is used in,
2073for instance, the C<use strict> pragma.
2074
2075The contents should be an integer; different bits of it are used for
241a59d9 2076different pragmatic flags. Here's an example:
1fa81471 2077
9548c15c 2078 sub add_100 { $^H |= 0x100 }
1fa81471 2079
9548c15c
FC
2080 sub foo {
2081 BEGIN { add_100() }
2082 bar->baz($boon);
2083 }
1fa81471 2084
241a59d9 2085Consider what happens during execution of the BEGIN block. At this point
1fa81471 2086the BEGIN block has already been compiled, but the body of C<foo()> is still
241a59d9
FC
2087being compiled. The new value of C<$^H>
2088will therefore be visible only while
1fa81471
DR
2089the body of C<foo()> is being compiled.
2090
2091Substitution of C<BEGIN { add_100() }> block with:
2092
9548c15c 2093 BEGIN { require strict; strict->import('vars') }
1fa81471 2094
241a59d9 2095demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1fa81471
DR
2096version of the same lexical pragma:
2097
9548c15c
FC
2098 BEGIN {
2099 require strict; strict->import('vars') if $condition
2100 }
1fa81471
DR
2101
2102This variable was added in Perl 5.003.
2103
2104=item %^H
2105X<%^H>
2106
241a59d9
FC
2107The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2108it useful for implementation of lexically scoped pragmas. See
112284f4
KW
2109L<perlpragma>. All the entries are stringified when accessed at
2110runtime, so only simple values can be accommodated. This means no
2111pointers to objects, for example.
1fa81471
DR
2112
2113When putting items into C<%^H>, in order to avoid conflicting with other
2114users of the hash there is a convention regarding which keys to use.
2115A module should use only keys that begin with the module's name (the
2116name of its main package) and a "/" character. For example, a module
2117C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2118
60cf4914 2119This variable was added in Perl v5.6.0.
1fa81471
DR
2120
2121=item ${^OPEN}
2122X<${^OPEN}>
2123
241a59d9 2124An internal variable used by PerlIO. A string in two parts, separated
1fa81471
DR
2125by a C<\0> byte, the first part describes the input layers, the second
2126part describes the output layers.
2127
60cf4914 2128This variable was added in Perl v5.8.0.
1fa81471
DR
2129
2130=item $PERLDB
2131
2132=item $^P
2133X<$^P> X<$PERLDB>
2134
241a59d9 2135The internal variable for debugging support. The meanings of the
1fa81471
DR
2136various bits are subject to change, but currently indicate:
2137
2138=over 6
2139
2140=item 0x01
2141
2142Debug subroutine enter/exit.
2143
2144=item 0x02
2145
241a59d9
FC
2146Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2147each statement executed. Also causes saving source code lines (like
21480x400).
1fa81471
DR
2149
2150=item 0x04
2151
2152Switch off optimizations.
2153
2154=item 0x08
2155
2156Preserve more data for future interactive inspections.
2157
2158=item 0x10
2159
2160Keep info about source lines on which a subroutine is defined.
2161
2162=item 0x20
2163
2164Start with single-step on.
2165
2166=item 0x40
2167
2168Use subroutine address instead of name when reporting.
2169
2170=item 0x80
2171
2172Report C<goto &subroutine> as well.
2173
2174=item 0x100
2175
2176Provide informative "file" names for evals based on the place they were compiled.
2177
2178=item 0x200
2179
2180Provide informative names to anonymous subroutines based on the place they
2181were compiled.
2182
2183=item 0x400
2184
2185Save source code lines into C<@{"_<$filename"}>.
2186
aab47982
RS
2187=item 0x800
2188
2189When saving source, include evals that generate no subroutines.
2190
2191=item 0x1000
2192
2193When saving source, include source that did not compile.
2194
1fa81471
DR
2195=back
2196
2197Some bits may be relevant at compile-time only, some at
241a59d9 2198run-time only. This is a new mechanism and the details may change.
1fa81471
DR
2199See also L<perldebguts>.
2200
2201=item ${^TAINT}
2202X<${^TAINT}>
2203
241a59d9 2204Reflects if taint mode is on or off. 1 for on (the program was run with
1fa81471
DR
2205B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2206B<-t> or B<-TU>).
2207
2208This variable is read-only.
2209
60cf4914 2210This variable was added in Perl v5.8.0.
1fa81471
DR
2211
2212=item ${^UNICODE}
2213X<${^UNICODE}>
2214
241a59d9 2215Reflects certain Unicode settings of Perl. See L<perlrun>
1fa81471
DR
2216documentation for the C<-C> switch for more information about
2217the possible values.
2218
2219This variable is set during Perl startup and is thereafter read-only.
2220
60cf4914 2221This variable was added in Perl v5.8.2.
1fa81471
DR
2222
2223=item ${^UTF8CACHE}
2224X<${^UTF8CACHE}>
2225
2226This variable controls the state of the internal UTF-8 offset caching code.
22271 for on (the default), 0 for off, -1 to debug the caching code by checking
2228all its results against linear scans, and panicking on any discrepancy.
2229
94df5432
KW
2230This variable was added in Perl v5.8.9. It is subject to change or
2231removal without notice, but is currently used to avoid recalculating the
2232boundaries of multi-byte UTF-8-encoded characters.
1fa81471
DR
2233
2234=item ${^UTF8LOCALE}
2235X<${^UTF8LOCALE}>
2236
2237This variable indicates whether a UTF-8 locale was detected by perl at
241a59d9 2238startup. This information is used by perl when it's in
1fa81471
DR
2239adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2240switch); see L<perlrun> for more info on this.
2241
60cf4914 2242This variable was added in Perl v5.8.8.
1fa81471
DR
2243
2244=back
2245
b0c22438 2246=head2 Deprecated and removed variables
7f315d2e 2247
0b9346e6 2248Deprecating a variable announces the intent of the perl maintainers to
241a59d9
FC
2249eventually remove the variable from the language. It may still be
2250available despite its status. Using a deprecated variable triggers
b0c22438 2251a warning.
7f315d2e 2252
84dabc03 2253Once a variable is removed, its use triggers an error telling you
b0c22438 2254the variable is unsupported.
7f315d2e 2255
84dabc03 2256See L<perldiag> for details about error messages.
7f315d2e 2257
b0c22438 2258=over 8
7f315d2e 2259
84dabc03 2260=item $#
b7a15f05 2261X<$#>
84dabc03 2262
38e5787b 2263C<$#> was a variable that could be used to format printed numbers.
60cf4914 2264After a deprecation cycle, its magic was removed in Perl v5.10.0 and
84dabc03 2265using it now triggers a warning: C<$# is no longer supported>.
2266
2267This is not the sigil you use in front of an array name to get the
241a59d9
FC
2268last index, like C<$#array>. That's still how you get the last index
2269of an array in Perl. The two have nothing to do with each other.
84dabc03 2270
2271Deprecated in Perl 5.
2272
60cf4914 2273Removed in Perl v5.10.0.
84dabc03 2274
7f315d2e
CO
2275=item $*
2276X<$*>
2277
84dabc03 2278C<$*> was a variable that you could use to enable multiline matching.
60cf4914 2279After a deprecation cycle, its magic was removed in Perl v5.10.0.
7f315d2e 2280Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2281You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2282
b0c22438 2283Deprecated in Perl 5.
7f315d2e 2284
60cf4914 2285Removed in Perl v5.10.0.
7f315d2e 2286
84dabc03 2287=item $[
b7a15f05 2288X<$[>
84dabc03 2289
b82b06b8
FC
2290This variable stores the index of the first element in an array, and
2291of the first character in a substring. The default is 0, but you could
2292theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2293when subscripting and when evaluating the index() and substr() functions.
84dabc03 2294
b82b06b8
FC
2295As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2296directive, and cannot influence the behavior of any other file.
2297(That's why you can only assign compile-time constants to it.)
2298Its use is highly discouraged.
2299
60cf4914 2300Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
b82b06b8
FC
2301scopes in the same file, unlike other compile-time directives (such as
2302L<strict>). Using local() on it would bind its value strictly to a lexical
2303block. Now it is always lexically scoped.
2304
60cf4914 2305As of Perl v5.16.0, it is implemented by the L<arybase> module. See
b82b06b8 2306L<arybase> for more details on its behaviour.
84dabc03 2307
6b54f8ab
FC
2308Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2309effect, and always contains 0. Assigning 0 to it is permitted, but any
2310other value will produce an error.
2311
b82b06b8
FC
2312Mnemonic: [ begins subscripts.
2313
60cf4914 2314Deprecated in Perl v5.12.0.
e1dccc0d 2315
b0c22438 2316=back
2b92dfce 2317
0b9346e6 2318=cut