3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<< <FH> >>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
116 Contains the subpattern from the corresponding set of capturing
117 parentheses from the last pattern match, not counting patterns
118 matched in nested blocks that have been exited already. (Mnemonic:
119 like \digits.) These variables are all read-only and dynamically
120 scoped to the current BLOCK.
126 The string matched by the last successful pattern match (not counting
127 any matches hidden within a BLOCK or eval() enclosed by the current
128 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
129 and dynamically scoped to the current BLOCK.
131 The use of this variable anywhere in a program imposes a considerable
132 performance penalty on all regular expression matches. See L<BUGS>.
138 The string preceding whatever was matched by the last successful
139 pattern match (not counting any matches hidden within a BLOCK or eval
140 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
141 string.) This variable is read-only.
143 The use of this variable anywhere in a program imposes a considerable
144 performance penalty on all regular expression matches. See L<BUGS>.
150 The string following whatever was matched by the last successful
151 pattern match (not counting any matches hidden within a BLOCK or eval()
152 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
157 print "$`:$&:$'\n"; # prints abc:def:ghi
159 This variable is read-only and dynamically scoped to the current BLOCK.
161 The use of this variable anywhere in a program imposes a considerable
162 performance penalty on all regular expression matches. See L<BUGS>.
164 =item $LAST_PAREN_MATCH
168 The last bracket matched by the last search pattern. This is useful if
169 you don't know which one of a set of alternative patterns matched. For
172 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
174 (Mnemonic: be positive and forward looking.)
175 This variable is read-only and dynamically scoped to the current BLOCK.
177 =item @LAST_MATCH_END
181 This array holds the offsets of the ends of the last successful
182 submatches in the currently active dynamic scope. C<$+[0]> is
183 the offset into the string of the end of the entire match. This
184 is the same value as what the C<pos> function returns when called
185 on the variable that was matched against. The I<n>th element
186 of this array holds the offset of the I<n>th submatch, so
187 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
188 past where $2 ends, and so on. You can use C<$#+> to determine
189 how many subgroups were in the last successful match. See the
190 examples given for the C<@-> variable.
192 =item $MULTILINE_MATCHING
196 Set to 1 to do multi-line matching within a string, 0 to tell Perl
197 that it can assume that strings contain a single line, for the purpose
198 of optimizing pattern matches. Pattern matches on strings containing
199 multiple newlines can produce confusing results when C<$*> is 0. Default
200 is 0. (Mnemonic: * matches multiple things.) This variable
201 influences the interpretation of only C<^> and C<$>. A literal newline can
202 be searched for even when C<$* == 0>.
204 Use of C<$*> is deprecated in modern Perl, supplanted by
205 the C</s> and C</m> modifiers on pattern matching.
207 =item input_line_number HANDLE EXPR
209 =item $INPUT_LINE_NUMBER
215 The current input record number for the last file handle from which
216 you just read() (or called a C<seek> or C<tell> on). The value
217 may be different from the actual physical line number in the file,
218 depending on what notion of "line" is in effect--see C<$/> on how
219 to change that. An explicit close on a filehandle resets the line
220 number. Because C<< <> >> never does an explicit close, line
221 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
222 Consider this variable read-only: setting it does not reposition
223 the seek pointer; you'll have to do that on your own. Localizing C<$.>
224 has the effect of also localizing Perl's notion of "the last read
225 filehandle". (Mnemonic: many programs use "." to mean the current line
228 =item input_record_separator HANDLE EXPR
230 =item $INPUT_RECORD_SEPARATOR
236 The input record separator, newline by default. This
237 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
238 variable, including treating empty lines as a terminator if set to
239 the null string. (An empty line cannot contain any spaces
240 or tabs.) You may set it to a multi-character string to match a
241 multi-character terminator, or to C<undef> to read through the end
242 of file. Setting it to C<"\n\n"> means something slightly
243 different than setting to C<"">, if the file contains consecutive
244 empty lines. Setting to C<""> will treat two or more consecutive
245 empty lines as a single empty line. Setting to C<"\n\n"> will
246 blindly assume that the next input character belongs to the next
247 paragraph, even if it's a newline. (Mnemonic: / delimits
248 line boundaries when quoting poetry.)
250 undef $/; # enable "slurp" mode
251 $_ = <FH>; # whole file now here
254 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
255 better for something. :-)
257 Setting C<$/> to a reference to an integer, scalar containing an integer, or
258 scalar that's convertible to an integer will attempt to read records
259 instead of lines, with the maximum record size being the referenced
262 $/ = \32768; # or \"32768", or \$var_containing_32768
266 will read a record of no more than 32768 bytes from FILE. If you're
267 not reading from a record-oriented file (or your OS doesn't have
268 record-oriented files), then you'll likely get a full chunk of data
269 with every read. If a record is larger than the record size you've
270 set, you'll get the record back in pieces.
272 On VMS, record reads are done with the equivalent of C<sysread>,
273 so it's best not to mix record and non-record reads on the same
274 file. (This is unlikely to be a problem, because any file you'd
275 want to read in record mode is probably unusable in line mode.)
276 Non-VMS systems do normal I/O, so it's safe to mix record and
277 non-record reads of a file.
279 See also L<perlport/"Newlines">. Also see C<$.>.
281 =item autoflush HANDLE EXPR
283 =item $OUTPUT_AUTOFLUSH
287 If set to nonzero, forces a flush right away and after every write
288 or print on the currently selected output channel. Default is 0
289 (regardless of whether the channel is really buffered by the
290 system or not; C<$|> tells you only whether you've asked Perl
291 explicitly to flush after each write). STDOUT will
292 typically be line buffered if output is to the terminal and block
293 buffered otherwise. Setting this variable is useful primarily when
294 you are outputting to a pipe or socket, such as when you are running
295 a Perl program under B<rsh> and want to see the output as it's
296 happening. This has no effect on input buffering. See L<perlfunc/getc>
297 for that. (Mnemonic: when you want your pipes to be piping hot.)
299 =item output_field_separator HANDLE EXPR
301 =item $OUTPUT_FIELD_SEPARATOR
307 The output field separator for the print operator. Ordinarily the
308 print operator simply prints out its arguments without further
309 adornment. To get behavior more like B<awk>, set this variable as
310 you would set B<awk>'s OFS variable to specify what is printed
311 between fields. (Mnemonic: what is printed when there is a "," in
312 your print statement.)
314 =item output_record_separator HANDLE EXPR
316 =item $OUTPUT_RECORD_SEPARATOR
322 The output record separator for the print operator. Ordinarily the
323 print operator simply prints out its arguments as is, with no
324 trailing newline or other end-of-record string added. To get
325 behavior more like B<awk>, set this variable as you would set
326 B<awk>'s ORS variable to specify what is printed at the end of the
327 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
328 end of the print. Also, it's just like C<$/>, but it's what you
329 get "back" from Perl.)
331 =item $LIST_SEPARATOR
335 This is like C<$,> except that it applies to array and slice values
336 interpolated into a double-quoted string (or similar interpreted
337 string). Default is a space. (Mnemonic: obvious, I think.)
339 =item $SUBSCRIPT_SEPARATOR
345 The subscript separator for multidimensional array emulation. If you
346 refer to a hash element as
352 $foo{join($;, $a, $b, $c)}
356 @foo{$a,$b,$c} # a slice--note the @
360 ($foo{$a},$foo{$b},$foo{$c})
362 Default is "\034", the same as SUBSEP in B<awk>. If your
363 keys contain binary data there might not be any safe value for C<$;>.
364 (Mnemonic: comma (the syntactic subscript separator) is a
365 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
366 taken for something more important.)
368 Consider using "real" multidimensional arrays as described
375 The output format for printed numbers. This variable is a half-hearted
376 attempt to emulate B<awk>'s OFMT variable. There are times, however,
377 when B<awk> and Perl have differing notions of what counts as
378 numeric. The initial value is "%.I<n>g", where I<n> is the value
379 of the macro DBL_DIG from your system's F<float.h>. This is different from
380 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
381 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
383 Use of C<$#> is deprecated.
385 =item format_page_number HANDLE EXPR
387 =item $FORMAT_PAGE_NUMBER
391 The current page number of the currently selected output channel.
393 (Mnemonic: % is page number in B<nroff>.)
395 =item format_lines_per_page HANDLE EXPR
397 =item $FORMAT_LINES_PER_PAGE
401 The current page length (printable lines) of the currently selected
402 output channel. Default is 60.
404 (Mnemonic: = has horizontal lines.)
406 =item format_lines_left HANDLE EXPR
408 =item $FORMAT_LINES_LEFT
412 The number of lines left on the page of the currently selected output
415 (Mnemonic: lines_on_page - lines_printed.)
417 =item @LAST_MATCH_START
421 $-[0] is the offset of the start of the last successful match.
422 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
423 I<n>-th subpattern, or undef if the subpattern did not match.
425 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
426 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
427 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
428 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
429 matched subgroup in the last successful match. Contrast with
430 C<$#+>, the number of subgroups in the regular expression. Compare
433 This array holds the offsets of the beginnings of the last
434 successful submatches in the currently active dynamic scope.
435 C<$-[0]> is the offset into the string of the beginning of the
436 entire match. The I<n>th element of this array holds the offset
437 of the I<n>th submatch, so C<$+[1]> is the offset where $1
438 begins, C<$+[2]> the offset where $2 begins, and so on.
439 You can use C<$#-> to determine how many subgroups were in the
440 last successful match. Compare with the C<@+> variable.
442 After a match against some variable $var:
446 =item C<$`> is the same as C<substr($var, 0, $-[0])>
448 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
450 =item C<$'> is the same as C<substr($var, $+[0])>
452 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
454 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
456 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
460 =item format_name HANDLE EXPR
466 The name of the current report format for the currently selected output
467 channel. Default is the name of the filehandle. (Mnemonic: brother to
470 =item format_top_name HANDLE EXPR
472 =item $FORMAT_TOP_NAME
476 The name of the current top-of-page format for the currently selected
477 output channel. Default is the name of the filehandle with _TOP
478 appended. (Mnemonic: points to top of page.)
480 =item format_line_break_characters HANDLE EXPR
482 =item $FORMAT_LINE_BREAK_CHARACTERS
486 The current set of characters after which a string may be broken to
487 fill continuation fields (starting with ^) in a format. Default is
488 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
489 poetry is a part of a line.)
491 =item format_formfeed HANDLE EXPR
493 =item $FORMAT_FORMFEED
497 What formats output as a form feed. Default is \f.
503 The current value of the write() accumulator for format() lines. A format
504 contains formline() calls that put their result into C<$^A>. After
505 calling its format, write() prints out the contents of C<$^A> and empties.
506 So you never really see the contents of C<$^A> unless you call
507 formline() yourself and then look at it. See L<perlform> and
508 L<perlfunc/formline()>.
514 The status returned by the last pipe close, backtick (C<``>) command,
515 successful call to wait() or waitpid(), or from the system()
516 operator. This is just the 16-bit status word returned by the
517 wait() system call (or else is made up to look like it). Thus, the
518 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
519 C<$? & 127> gives which signal, if any, the process died from, and
520 C<$? & 128> reports whether there was a core dump. (Mnemonic:
521 similar to B<sh> and B<ksh>.)
523 Additionally, if the C<h_errno> variable is supported in C, its value
524 is returned via $? if any C<gethost*()> function fails.
526 If you have installed a signal handler for C<SIGCHLD>, the
527 value of C<$?> will usually be wrong outside that handler.
529 Inside an C<END> subroutine C<$?> contains the value that is going to be
530 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
531 change the exit status of your program. For example:
534 $? = 1 if $? == 255; # die would make it 255
537 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
538 actual VMS exit status, instead of the default emulation of POSIX
541 Also see L<Error Indicators>.
549 If used numerically, yields the current value of the C C<errno>
550 variable, with all the usual caveats. (This means that you shouldn't
551 depend on the value of C<$!> to be anything in particular unless
552 you've gotten a specific error return indicating a system error.)
553 If used an a string, yields the corresponding system error string.
554 You can assign a number to C<$!> to set I<errno> if, for instance,
555 you want C<"$!"> to return the string for error I<n>, or you want
556 to set the exit value for the die() operator. (Mnemonic: What just
559 Also see L<Error Indicators>.
561 =item $EXTENDED_OS_ERROR
565 Error information specific to the current operating system. At
566 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
567 (and for MacPerl). On all other platforms, C<$^E> is always just
570 Under VMS, C<$^E> provides the VMS status value from the last
571 system error. This is more specific information about the last
572 system error than that provided by C<$!>. This is particularly
573 important when C<$!> is set to B<EVMSERR>.
575 Under OS/2, C<$^E> is set to the error code of the last call to
576 OS/2 API either via CRT, or directly from perl.
578 Under Win32, C<$^E> always returns the last error information
579 reported by the Win32 call C<GetLastError()> which describes
580 the last error from within the Win32 API. Most Win32-specific
581 code will report errors via C<$^E>. ANSI C and Unix-like calls
582 set C<errno> and so most portable Perl code will report errors
585 Caveats mentioned in the description of C<$!> generally apply to
586 C<$^E>, also. (Mnemonic: Extra error explanation.)
588 Also see L<Error Indicators>.
594 The Perl syntax error message from the last eval() operator. If null, the
595 last eval() parsed and executed correctly (although the operations you
596 invoked may have failed in the normal fashion). (Mnemonic: Where was
597 the syntax error "at"?)
599 Warning messages are not collected in this variable. You can,
600 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
603 Also see L<Error Indicators>.
611 The process number of the Perl running this script. You should
612 consider this variable read-only, although it will be altered
613 across fork() calls. (Mnemonic: same as shells.)
621 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
622 if you're running setuid.) You can change both the real uid and
623 the effective uid at the same time by using POSIX::setuid().
625 =item $EFFECTIVE_USER_ID
631 The effective uid of this process. Example:
633 $< = $>; # set real to effective uid
634 ($<,$>) = ($>,$<); # swap real and effective uid
636 You can change both the effective uid and the real uid at the same
637 time by using POSIX::setuid().
639 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
640 C<< $< >> and C<< $> >> can be swapped only on machines
641 supporting setreuid().
649 The real gid of this process. If you are on a machine that supports
650 membership in multiple groups simultaneously, gives a space separated
651 list of groups you are in. The first number is the one returned by
652 getgid(), and the subsequent ones by getgroups(), one of which may be
653 the same as the first number.
655 However, a value assigned to C<$(> must be a single number used to
656 set the real gid. So the value given by C<$(> should I<not> be assigned
657 back to C<$(> without being forced numeric, such as by adding zero.
659 You can change both the real gid and the effective gid at the same
660 time by using POSIX::setgid().
662 (Mnemonic: parentheses are used to I<group> things. The real gid is the
663 group you I<left>, if you're running setgid.)
665 =item $EFFECTIVE_GROUP_ID
671 The effective gid of this process. If you are on a machine that
672 supports membership in multiple groups simultaneously, gives a space
673 separated list of groups you are in. The first number is the one
674 returned by getegid(), and the subsequent ones by getgroups(), one of
675 which may be the same as the first number.
677 Similarly, a value assigned to C<$)> must also be a space-separated
678 list of numbers. The first number sets the effective gid, and
679 the rest (if any) are passed to setgroups(). To get the effect of an
680 empty list for setgroups(), just repeat the new effective gid; that is,
681 to force an effective gid of 5 and an effectively empty setgroups()
682 list, say C< $) = "5 5" >.
684 You can change both the effective gid and the real gid at the same
685 time by using POSIX::setgid() (use only a single numeric argument).
687 (Mnemonic: parentheses are used to I<group> things. The effective gid
688 is the group that's I<right> for you, if you're running setgid.)
690 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
691 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
692 and C<$)> can be swapped only on machines supporting setregid().
698 Contains the name of the program being executed. On some operating
699 systems assigning to C<$0> modifies the argument area that the B<ps>
700 program sees. This is more useful as a way of indicating the current
701 program state than it is for hiding the program you're running.
702 (Mnemonic: same as B<sh> and B<ksh>.)
704 Note for BSD users: setting C<$0> does not completely remove "perl"
705 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
706 result in C<"perl: foobar (perl)">. This is an operating system
711 The index of the first element in an array, and of the first character
712 in a substring. Default is 0, but you could theoretically set it
713 to 1 to make Perl behave more like B<awk> (or Fortran) when
714 subscripting and when evaluating the index() and substr() functions.
715 (Mnemonic: [ begins subscripts.)
717 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
718 directive, and cannot influence the behavior of any other file.
719 Its use is highly discouraged.
723 The version + patchlevel / 1000 of the Perl interpreter. This variable
724 can be used to determine whether the Perl interpreter executing a
725 script is in the right range of versions. (Mnemonic: Is this version
726 of perl in the right bracket?) Example:
728 warn "No checksumming!\n" if $] < 3.019;
730 See also the documentation of C<use VERSION> and C<require VERSION>
731 for a convenient way to fail if the running Perl interpreter is too old.
733 The use of this variable is deprecated. The floating point representation
734 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
735 more modern representation of the Perl version that allows accurate string
742 The current value of the flag associated with the B<-c> switch.
743 Mainly of use with B<-MO=...> to allow code to alter its behavior
744 when being compiled, such as for example to AUTOLOAD at compile
745 time rather than normal, deferred loading. See L<perlcc>. Setting
746 C<$^C = 1> is similar to calling C<B::minus_c>.
752 The current value of the debugging flags. (Mnemonic: value of B<-D>
759 The maximum system file descriptor, ordinarily 2. System file
760 descriptors are passed to exec()ed processes, while higher file
761 descriptors are not. Also, during an open(), system file descriptors are
762 preserved even if the open() fails. (Ordinary file descriptors are
763 closed before the open() is attempted.) The close-on-exec
764 status of a file descriptor will be decided according to the value of
765 C<$^F> when the corresponding file, pipe, or socket was opened, not the
770 WARNING: This variable is strictly for internal use only. Its availability,
771 behavior, and contents are subject to change without notice.
773 This variable contains compile-time hints for the Perl interpreter. At the
774 end of compilation of a BLOCK the value of this variable is restored to the
775 value when the interpreter started to compile the BLOCK.
777 When perl begins to parse any block construct that provides a lexical scope
778 (e.g., eval body, required file, subroutine body, loop body, or conditional
779 block), the existing value of $^H is saved, but its value is left unchanged.
780 When the compilation of the block is completed, it regains the saved value.
781 Between the points where its value is saved and restored, code that
782 executes within BEGIN blocks is free to change the value of $^H.
784 This behavior provides the semantic of lexical scoping, and is used in,
785 for instance, the C<use strict> pragma.
787 The contents should be an integer; different bits of it are used for
788 different pragmatic flags. Here's an example:
790 sub add_100 { $^H |= 0x100 }
797 Consider what happens during execution of the BEGIN block. At this point
798 the BEGIN block has already been compiled, but the body of foo() is still
799 being compiled. The new value of $^H will therefore be visible only while
800 the body of foo() is being compiled.
802 Substitution of the above BEGIN block with:
804 BEGIN { require strict; strict->import('vars') }
806 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
807 version of the same lexical pragma:
809 BEGIN { require strict; strict->import('vars') if $condition }
813 WARNING: This variable is strictly for internal use only. Its availability,
814 behavior, and contents are subject to change without notice.
816 The %^H hash provides the same scoping semantic as $^H. This makes it
817 useful for implementation of lexically scoped pragmas.
823 The current value of the inplace-edit extension. Use C<undef> to disable
824 inplace editing. (Mnemonic: value of B<-i> switch.)
828 By default, running out of memory is an untrappable, fatal error.
829 However, if suitably built, Perl can use the contents of C<$^M>
830 as an emergency memory pool after die()ing. Suppose that your Perl
831 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
834 $^M = 'a' x (1 << 16);
836 would allocate a 64K buffer for use in an emergency. See the
837 F<INSTALL> file in the Perl distribution for information on how to
838 enable this option. To discourage casual use of this advanced
839 feature, there is no L<English|English> long name for this variable.
845 The name of the operating system under which this copy of Perl was
846 built, as determined during the configuration process. The value
847 is identical to C<$Config{'osname'}>. See also L<Config> and the
848 B<-V> command-line switch documented in L<perlrun>.
854 The internal variable for debugging support. The meanings of the
855 various bits are subject to change, but currently indicate:
861 Debug subroutine enter/exit.
865 Line-by-line debugging.
869 Switch off optimizations.
873 Preserve more data for future interactive inspections.
877 Keep info about source lines on which a subroutine is defined.
881 Start with single-step on.
885 Use subroutine address instead of name when reporting.
889 Report C<goto &subroutine> as well.
893 Provide informative "file" names for evals based on the place they were compiled.
897 Provide informative names to anonymous subroutines based on the place they
902 Some bits may be relevant at compile-time only, some at
903 run-time only. This is a new mechanism and the details may change.
905 =item $LAST_REGEXP_CODE_RESULT
909 The result of evaluation of the last successful C<(?{ code })>
910 regular expression assertion (see L<perlre>). May be written to.
912 =item $EXCEPTIONS_BEING_CAUGHT
916 Current state of the interpreter. Undefined if parsing of the current
917 module/eval is not finished (may happen in $SIG{__DIE__} and
918 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
924 The time at which the program began running, in seconds since the
925 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
926 and B<-C> filetests are based on this value.
932 The revision, version, and subversion of the Perl interpreter, represented
933 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
934 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
935 C<$^V eq v5.6.0>. Note that the characters in this string value can
936 potentially be in Unicode range.
938 This can be used to determine whether the Perl interpreter executing a
939 script is in the right range of versions. (Mnemonic: use ^V for Version
942 warn "No "our" declarations!\n" if $^V and $^V lt v5.6.0;
944 See the documentation of C<use VERSION> and C<require VERSION>
945 for a convenient way to fail if the running Perl interpreter is too old.
947 See also C<$]> for an older representation of the Perl version.
953 The current value of the warning switch, initially true if B<-w>
954 was used, false otherwise, but directly modifiable. (Mnemonic:
955 related to the B<-w> switch.) See also L<warnings>.
957 =item ${^WARNING_BITS}
959 The current set of warning checks enabled by the C<use warnings> pragma.
960 See the documentation of C<warnings> for more details.
962 =item ${^WIDE_SYSTEM_CALLS}
964 Global flag that enables system calls made by Perl to use wide character
965 APIs native to the system, if available. This is currently only implemented
966 on the Windows platform.
968 This can also be enabled from the command line using the C<-C> switch.
970 The initial value is typically C<0> for compatibility with Perl versions
971 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
972 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
974 The C<bytes> pragma always overrides the effect of this flag in the current
975 lexical scope. See L<bytes>.
977 =item $EXECUTABLE_NAME
981 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
982 This may not be a full pathname, nor even necessarily in your path.
986 contains the name of the current file when reading from <>.
990 The array @ARGV contains the command-line arguments intended for
991 the script. C<$#ARGV> is generally the number of arguments minus
992 one, because C<$ARGV[0]> is the first argument, I<not> the program's
993 command name itself. See C<$0> for the command name.
997 The array @INC contains the list of places that the C<do EXPR>,
998 C<require>, or C<use> constructs look for their library files. It
999 initially consists of the arguments to any B<-I> command-line
1000 switches, followed by the default Perl library, probably
1001 F</usr/local/lib/perl>, followed by ".", to represent the current
1002 directory. If you need to modify this at runtime, you should use
1003 the C<use lib> pragma to get the machine-dependent library properly
1006 use lib '/mypath/libdir/';
1011 Within a subroutine the array @_ contains the parameters passed to that
1012 subroutine. See L<perlsub>.
1016 The hash %INC contains entries for each filename included via the
1017 C<do>, C<require>, or C<use> operators. The key is the filename
1018 you specified (with module names converted to pathnames), and the
1019 value is the location of the file found. The C<require>
1020 operator uses this hash to determine whether a particular file has
1021 already been included.
1027 The hash %ENV contains your current environment. Setting a
1028 value in C<ENV> changes the environment for any child processes
1029 you subsequently fork() off.
1035 The hash %SIG contains signal handlers for signals. For example:
1037 sub handler { # 1st argument is signal name
1039 print "Caught a SIG$sig--shutting down\n";
1044 $SIG{'INT'} = \&handler;
1045 $SIG{'QUIT'} = \&handler;
1047 $SIG{'INT'} = 'DEFAULT'; # restore default action
1048 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1050 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1051 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1054 Here are some other examples:
1056 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1057 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1058 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1059 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1061 Be sure not to use a bareword as the name of a signal handler,
1062 lest you inadvertently call it.
1064 If your system has the sigaction() function then signal handlers are
1065 installed using it. This means you get reliable signal handling. If
1066 your system has the SA_RESTART flag it is used when signals handlers are
1067 installed. This means that system calls for which restarting is supported
1068 continue rather than returning when a signal arrives. If you want your
1069 system calls to be interrupted by signal delivery then do something like
1072 use POSIX ':signal_h';
1075 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1076 or die "Error setting SIGALRM handler: $!\n";
1080 Certain internal hooks can be also set using the %SIG hash. The
1081 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1082 about to be printed. The warning message is passed as the first
1083 argument. The presence of a __WARN__ hook causes the ordinary printing
1084 of warnings to STDERR to be suppressed. You can use this to save warnings
1085 in a variable, or turn warnings into fatal errors, like this:
1087 local $SIG{__WARN__} = sub { die $_[0] };
1090 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1091 is about to be thrown. The error message is passed as the first
1092 argument. When a __DIE__ hook routine returns, the exception
1093 processing continues as it would have in the absence of the hook,
1094 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1095 The C<__DIE__> handler is explicitly disabled during the call, so that you
1096 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1098 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1099 even inside an eval(). Do not use this to rewrite a pending exception
1100 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1101 This strange action at a distance may be fixed in a future release
1102 so that C<$SIG{__DIE__}> is only called if your program is about
1103 to exit, as was the original intent. Any other use is deprecated.
1105 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1106 they may be called to report (probable) errors found by the parser.
1107 In such a case the parser may be in inconsistent state, so any
1108 attempt to evaluate Perl code from such a handler will probably
1109 result in a segfault. This means that warnings or errors that
1110 result from parsing Perl should be used with extreme caution, like
1113 require Carp if defined $^S;
1114 Carp::confess("Something wrong") if defined &Carp::confess;
1115 die "Something wrong, but could not load Carp to give backtrace...
1116 To see backtrace try starting Perl with -MCarp switch";
1118 Here the first line will load Carp I<unless> it is the parser who
1119 called the handler. The second line will print backtrace and die if
1120 Carp was available. The third line will be executed only if Carp was
1123 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1124 L<warnings> for additional information.
1128 =head2 Error Indicators
1130 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1131 about different types of error conditions that may appear during
1132 execution of a Perl program. The variables are shown ordered by
1133 the "distance" between the subsystem which reported the error and
1134 the Perl process. They correspond to errors detected by the Perl
1135 interpreter, C library, operating system, or an external program,
1138 To illustrate the differences between these variables, consider the
1139 following Perl expression, which uses a single-quoted string:
1142 open PIPE, "/cdrom/install |";
1144 close PIPE or die "bad pipe: $?, $!";
1147 After execution of this statement all 4 variables may have been set.
1149 C<$@> is set if the string to be C<eval>-ed did not compile (this
1150 may happen if C<open> or C<close> were imported with bad prototypes),
1151 or if Perl code executed during evaluation die()d . In these cases
1152 the value of $@ is the compile error, or the argument to C<die>
1153 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1156 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1157 and C<close> are translated to calls in the C run-time library and
1158 thence to the operating system kernel. C<$!> is set to the C library's
1159 C<errno> if one of these calls fails.
1161 Under a few operating systems, C<$^E> may contain a more verbose
1162 error indicator, such as in this case, "CDROM tray not closed."
1163 Systems that do not support extended error messages leave C<$^E>
1166 Finally, C<$?> may be set to non-0 value if the external program
1167 F</cdrom/install> fails. The upper eight bits reflect specific
1168 error conditions encountered by the program (the program's exit()
1169 value). The lower eight bits reflect mode of failure, like signal
1170 death and core dump information See wait(2) for details. In
1171 contrast to C<$!> and C<$^E>, which are set only if error condition
1172 is detected, the variable C<$?> is set on each C<wait> or pipe
1173 C<close>, overwriting the old value. This is more like C<$@>, which
1174 on every eval() is always set on failure and cleared on success.
1176 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1179 =head2 Technical Note on the Syntax of Variable Names
1181 Variable names in Perl can have several formats. Usually, they
1182 must begin with a letter or underscore, in which case they can be
1183 arbitrarily long (up to an internal limit of 251 characters) and
1184 may contain letters, digits, underscores, or the special sequence
1185 C<::> or C<'>. In this case, the part before the last C<::> or
1186 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1188 Perl variable names may also be a sequence of digits or a single
1189 punctuation or control character. These names are all reserved for
1190 special uses by Perl; for example, the all-digits names are used
1191 to hold data captured by backreferences after a regular expression
1192 match. Perl has a special syntax for the single-control-character
1193 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1194 character. For example, the notation C<$^W> (dollar-sign caret
1195 C<W>) is the scalar variable whose name is the single character
1196 control-C<W>. This is better than typing a literal control-C<W>
1199 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1200 strings that begin with control characters (or better yet, a caret).
1201 These variables must be written in the form C<${^Foo}>; the braces
1202 are not optional. C<${^Foo}> denotes the scalar variable whose
1203 name is a control-C<F> followed by two C<o>'s. These variables are
1204 reserved for future special uses by Perl, except for the ones that
1205 begin with C<^_> (control-underscore or caret-underscore). No
1206 control-character name that begins with C<^_> will acquire a special
1207 meaning in any future version of Perl; such names may therefore be
1208 used safely in programs. C<$^_> itself, however, I<is> reserved.
1210 Perl identifiers that begin with digits, control characters, or
1211 punctuation characters are exempt from the effects of the C<package>
1212 declaration and are always forced to be in package C<main>. A few
1213 other names are also exempt:
1221 In particular, the new special C<${^_XYZ}> variables are always taken
1222 to be in package C<main>, regardless of any C<package> declarations
1227 Due to an unfortunate accident of Perl's implementation, C<use
1228 English> imposes a considerable performance penalty on all regular
1229 expression matches in a program, regardless of whether they occur
1230 in the scope of C<use English>. For that reason, saying C<use
1231 English> in libraries is strongly discouraged. See the
1232 Devel::SawAmpersand module documentation from CPAN
1233 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1234 for more information.
1236 Having to even think about the C<$^S> variable in your exception
1237 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1238 invites grievous and difficult to track down errors. Avoid it
1239 and use an C<END{}> or CORE::GLOBAL::die override instead.