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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
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10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
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13
14 use English;
15
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16at the top of your program. This aliases all the short names to the long
17names in the current package. Some even have medium names, generally
18borrowed from B<awk>. In general, it's best to use the
a0d0e21e 19
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20 use English '-no_match_vars';
21
22invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23a certain performance hit with the use of regular expressions. See
24L<English>.
25
26Variables that depend on the currently selected filehandle may be set by
27calling an appropriate object method on the IO::Handle object, although
28this is less efficient than using the regular built-in variables. (Summary
29lines below for this contain the word HANDLE.) First you must say
a0d0e21e 30
19799a22 31 use IO::Handle;
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32
33after which you may use either
34
35 method HANDLE EXPR
36
5a964f20 37or more safely,
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38
39 HANDLE->method(EXPR)
40
14218588 41Each method returns the old value of the IO::Handle attribute.
a1ce9542 42The methods each take an optional EXPR, which, if supplied, specifies the
19799a22 43new value for the IO::Handle attribute in question. If not supplied,
14218588 44most methods do nothing to the current value--except for
a0d0e21e 45autoflush(), which will assume a 1 for you, just to be different.
a1ce9542 46
14218588 47Because loading in the IO::Handle class is an expensive operation, you should
19799a22 48learn how to use the regular built-in variables.
a0d0e21e 49
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50A few of these variables are considered "read-only". This means that if
51you try to assign to this variable, either directly or indirectly through
52a reference, you'll raise a run-time exception.
a0d0e21e 53
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54You should be very careful when modifying the default values of most
55special variables described in this document. In most cases you want
56to localize these variables before changing them, since if you don't,
57the change may affect other modules which rely on the default values
58of the special variables that you have changed. This is one of the
59correct ways to read the whole file at once:
60
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
63 my $content = <$fh>;
64 close $fh;
65
66But the following code is quite bad:
67
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
70 my $content = <$fh>;
71 close $fh;
72
73since some other module, may want to read data from some file in the
74default "line mode", so if the code we have just presented has been
75executed, the global value of C<$/> is now changed for any other code
76running inside the same Perl interpreter.
77
78Usually when a variable is localized you want to make sure that this
79change affects the shortest scope possible. So unless you are already
80inside some short C<{}> block, you should create one yourself. For
81example:
82
83 my $content = '';
84 open my $fh, "foo" or die $!;
85 {
86 local $/;
87 $content = <$fh>;
88 }
89 close $fh;
90
91Here is an example of how your own code can go broken:
92
93 for (1..5){
94 nasty_break();
95 print "$_ ";
96 }
97 sub nasty_break {
98 $_ = 5;
99 # do something with $_
100 }
101
102You probably expect this code to print:
103
104 1 2 3 4 5
105
106but instead you get:
107
108 5 5 5 5 5
109
110Why? Because nasty_break() modifies C<$_> without localizing it
111first. The fix is to add local():
112
113 local $_ = 5;
114
115It's easy to notice the problem in such a short example, but in more
116complicated code you are looking for trouble if you don't localize
117changes to the special variables.
118
fb73857a 119The following list is ordered by scalar variables first, then the
87275199 120arrays, then the hashes.
fb73857a 121
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122=over 8
123
124=item $ARG
125
126=item $_
a054c801 127X<$_> X<$ARG>
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128
129The default input and pattern-searching space. The following pairs are
130equivalent:
131
19799a22 132 while (<>) {...} # equivalent only in while!
54310121 133 while (defined($_ = <>)) {...}
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134
135 /^Subject:/
136 $_ =~ /^Subject:/
137
138 tr/a-z/A-Z/
139 $_ =~ tr/a-z/A-Z/
140
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141 chomp
142 chomp($_)
a0d0e21e 143
54310121 144Here are the places where Perl will assume $_ even if you
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145don't use it:
146
147=over 3
148
149=item *
150
151Various unary functions, including functions like ord() and int(), as well
152as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
153STDIN.
154
155=item *
156
157Various list functions like print() and unlink().
158
159=item *
160
161The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
162without an C<=~> operator.
163
54310121 164=item *
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165
166The default iterator variable in a C<foreach> loop if no other
167variable is supplied.
168
54310121 169=item *
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170
171The implicit iterator variable in the grep() and map() functions.
172
54310121 173=item *
cb1a09d0 174
c47ff5f1 175The default place to put an input record when a C<< <FH> >>
cb1a09d0 176operation's result is tested by itself as the sole criterion of a C<while>
14218588 177test. Outside a C<while> test, this will not happen.
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178
179=back
180
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181As C<$_> is a global variable, this may lead in some cases to unwanted
182side-effects. As of perl 5.9.1, you can now use a lexical version of
183C<$_> by declaring it in a file or in a block with C<my>. Moreover,
184declaring C<our $> restores the global C<$_> in the current scope.
185
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186(Mnemonic: underline is understood in certain operations.)
187
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188=back
189
190=over 8
191
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192=item $a
193
194=item $b
a054c801 195X<$a> X<$b>
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196
197Special package variables when using sort(), see L<perlfunc/sort>.
198Because of this specialness $a and $b don't need to be declared
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199(using use vars, or our()) even when using the C<strict 'vars'> pragma.
200Don't lexicalize them with C<my $a> or C<my $b> if you want to be
201able to use them in the sort() comparison block or function.
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202
203=back
204
205=over 8
206
c47ff5f1 207=item $<I<digits>>
a054c801 208X<$1> X<$2> X<$3>
a0d0e21e 209
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210Contains the subpattern from the corresponding set of capturing
211parentheses from the last pattern match, not counting patterns
212matched in nested blocks that have been exited already. (Mnemonic:
213like \digits.) These variables are all read-only and dynamically
214scoped to the current BLOCK.
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215
216=item $MATCH
217
218=item $&
a054c801 219X<$&> X<$MATCH>
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220
221The string matched by the last successful pattern match (not counting
222any matches hidden within a BLOCK or eval() enclosed by the current
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223BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
224and dynamically scoped to the current BLOCK.
a0d0e21e 225
19ddd453 226The use of this variable anywhere in a program imposes a considerable
667e1aea 227performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 228
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229See L</@-> for a replacement.
230
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231=item ${^MATCH}
232X<${^MATCH}>
233
234This is similar to C<$&> (C<$POSTMATCH>) except that it does not incur the
235performance penalty associated with that variable, and is only guaranteed
236to return a defined value when the pattern was compiled or executed with
87e95b7f 237the C</p> modifier.
cde0cee5 238
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239=item $PREMATCH
240
241=item $`
a054c801 242X<$`> X<$PREMATCH>
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243
244The string preceding whatever was matched by the last successful
245pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 246enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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247string.) This variable is read-only.
248
19ddd453 249The use of this variable anywhere in a program imposes a considerable
667e1aea 250performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 251
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252See L</@-> for a replacement.
253
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254=item ${^PREMATCH}
255X<${^PREMATCH}>
256
257This is similar to C<$`> ($PREMATCH) except that it does not incur the
258performance penalty associated with that variable, and is only guaranteed
259to return a defined value when the pattern was compiled or executed with
87e95b7f 260the C</p> modifier.
cde0cee5 261
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262=item $POSTMATCH
263
264=item $'
a054c801 265X<$'> X<$POSTMATCH>
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266
267The string following whatever was matched by the last successful
268pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 269enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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270string.) Example:
271
22d0716c 272 local $_ = 'abcdefghi';
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273 /def/;
274 print "$`:$&:$'\n"; # prints abc:def:ghi
275
19799a22 276This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 277
19ddd453 278The use of this variable anywhere in a program imposes a considerable
667e1aea 279performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 280
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281See L</@-> for a replacement.
282
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283=item ${^POSTMATCH}
284X<${^POSTMATCH}>
285
286This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
287performance penalty associated with that variable, and is only guaranteed
288to return a defined value when the pattern was compiled or executed with
87e95b7f 289the C</p> modifier.
cde0cee5 290
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291=item $LAST_PAREN_MATCH
292
293=item $+
a054c801 294X<$+> X<$LAST_PAREN_MATCH>
a0d0e21e 295
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296The text matched by the last bracket of the last successful search pattern.
297This is useful if you don't know which one of a set of alternative patterns
298matched. For example:
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299
300 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
301
302(Mnemonic: be positive and forward looking.)
19799a22 303This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 304
a01268b5 305=item $^N
a054c801 306X<$^N>
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307
308The text matched by the used group most-recently closed (i.e. the group
309with the rightmost closing parenthesis) of the last successful search
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310pattern. (Mnemonic: the (possibly) Nested parenthesis that most
311recently closed.)
312
210b36aa 313This is primarily used inside C<(?{...})> blocks for examining text
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314recently matched. For example, to effectively capture text to a variable
315(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
316
317 (?:(...)(?{ $var = $^N }))
318
319By setting and then using C<$var> in this way relieves you from having to
320worry about exactly which numbered set of parentheses they are.
321
322This variable is dynamically scoped to the current BLOCK.
323
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324=item @LAST_MATCH_END
325
6cef1e77 326=item @+
a054c801 327X<@+> X<@LAST_MATCH_END>
6cef1e77 328
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329This array holds the offsets of the ends of the last successful
330submatches in the currently active dynamic scope. C<$+[0]> is
331the offset into the string of the end of the entire match. This
332is the same value as what the C<pos> function returns when called
333on the variable that was matched against. The I<n>th element
334of this array holds the offset of the I<n>th submatch, so
335C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
336past where $2 ends, and so on. You can use C<$#+> to determine
337how many subgroups were in the last successful match. See the
338examples given for the C<@-> variable.
6cef1e77 339
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340=item %+
341X<%+>
342
343Similar to C<@+>, the C<%+> hash allows access to the named capture
344buffers, should they exist, in the last successful match in the
345currently active dynamic scope.
346
347C<$+{foo}> is equivalent to C<$1> after the following match:
348
349 'foo'=~/(?<foo>foo)/;
350
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351The underlying behaviour of %+ is provided by the L<re::Tie::Hash::NamedCapture>
352module.
353
354B<Note:> As C<%-> and C<%+> are tied views into a common internal hash
355associated with the last successful regular expression. Therefore mixing
356iterative access to them via C<each> may have unpredictable results.
357Likewise, if the last successful match changes then the results may be
358surprising.
359
fcc7d916 360=item HANDLE->input_line_number(EXPR)
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361
362=item $INPUT_LINE_NUMBER
363
364=item $NR
365
366=item $.
a054c801 367X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
a0d0e21e 368
81714fb9 369Current line number for the last filehandle accessed.
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370
371Each filehandle in Perl counts the number of lines that have been read
372from it. (Depending on the value of C<$/>, Perl's idea of what
373constitutes a line may not match yours.) When a line is read from a
374filehandle (via readline() or C<< <> >>), or when tell() or seek() is
375called on it, C<$.> becomes an alias to the line counter for that
376filehandle.
377
378You can adjust the counter by assigning to C<$.>, but this will not
379actually move the seek pointer. I<Localizing C<$.> will not localize
380the filehandle's line count>. Instead, it will localize perl's notion
381of which filehandle C<$.> is currently aliased to.
382
383C<$.> is reset when the filehandle is closed, but B<not> when an open
384filehandle is reopened without an intervening close(). For more
e48df184 385details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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386an explicit close, line numbers increase across ARGV files (but see
387examples in L<perlfunc/eof>).
388
389You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
390line counter for a given filehandle without having to worry about
391which handle you last accessed.
392
393(Mnemonic: many programs use "." to mean the current line number.)
394
395=item IO::Handle->input_record_separator(EXPR)
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396
397=item $INPUT_RECORD_SEPARATOR
398
399=item $RS
400
401=item $/
a054c801 402X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
a0d0e21e 403
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404The input record separator, newline by default. This
405influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 406variable, including treating empty lines as a terminator if set to
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407the null string. (An empty line cannot contain any spaces
408or tabs.) You may set it to a multi-character string to match a
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409multi-character terminator, or to C<undef> to read through the end
410of file. Setting it to C<"\n\n"> means something slightly
411different than setting to C<"">, if the file contains consecutive
412empty lines. Setting to C<""> will treat two or more consecutive
413empty lines as a single empty line. Setting to C<"\n\n"> will
414blindly assume that the next input character belongs to the next
14218588 415paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 416line boundaries when quoting poetry.)
a0d0e21e 417
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418 local $/; # enable "slurp" mode
419 local $_ = <FH>; # whole file now here
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420 s/\n[ \t]+/ /g;
421
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422Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
423better for something. :-)
68dc0745 424
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425Setting C<$/> to a reference to an integer, scalar containing an integer, or
426scalar that's convertible to an integer will attempt to read records
5b2b9c68 427instead of lines, with the maximum record size being the referenced
19799a22 428integer. So this:
5b2b9c68 429
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430 local $/ = \32768; # or \"32768", or \$var_containing_32768
431 open my $fh, $myfile or die $!;
432 local $_ = <$fh>;
5b2b9c68 433
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434will read a record of no more than 32768 bytes from FILE. If you're
435not reading from a record-oriented file (or your OS doesn't have
436record-oriented files), then you'll likely get a full chunk of data
437with every read. If a record is larger than the record size you've
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438set, you'll get the record back in pieces. Trying to set the record
439size to zero or less will cause reading in the (rest of the) whole file.
5b2b9c68 440
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441On VMS, record reads are done with the equivalent of C<sysread>,
442so it's best not to mix record and non-record reads on the same
443file. (This is unlikely to be a problem, because any file you'd
83763826 444want to read in record mode is probably unusable in line mode.)
14218588 445Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 446non-record reads of a file.
5b2b9c68 447
14218588 448See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 449
fcc7d916 450=item HANDLE->autoflush(EXPR)
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451
452=item $OUTPUT_AUTOFLUSH
453
454=item $|
a054c801 455X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
a0d0e21e 456
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457If set to nonzero, forces a flush right away and after every write
458or print on the currently selected output channel. Default is 0
14218588 459(regardless of whether the channel is really buffered by the
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460system or not; C<$|> tells you only whether you've asked Perl
461explicitly to flush after each write). STDOUT will
462typically be line buffered if output is to the terminal and block
463buffered otherwise. Setting this variable is useful primarily when
464you are outputting to a pipe or socket, such as when you are running
465a Perl program under B<rsh> and want to see the output as it's
466happening. This has no effect on input buffering. See L<perlfunc/getc>
467for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 468
46550894 469=item IO::Handle->output_field_separator EXPR
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470
471=item $OUTPUT_FIELD_SEPARATOR
472
473=item $OFS
474
475=item $,
a054c801 476X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
a0d0e21e 477
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478The output field separator for the print operator. If defined, this
479value is printed between each of print's arguments. Default is C<undef>.
480(Mnemonic: what is printed when there is a "," in your print statement.)
a0d0e21e 481
46550894 482=item IO::Handle->output_record_separator EXPR
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483
484=item $OUTPUT_RECORD_SEPARATOR
485
486=item $ORS
487
488=item $\
a054c801 489X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
a0d0e21e 490
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491The output record separator for the print operator. If defined, this
492value is printed after the last of print's arguments. Default is C<undef>.
493(Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
494Also, it's just like C<$/>, but it's what you get "back" from Perl.)
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495
496=item $LIST_SEPARATOR
497
498=item $"
a054c801 499X<$"> X<$LIST_SEPARATOR>
a0d0e21e 500
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501This is like C<$,> except that it applies to array and slice values
502interpolated into a double-quoted string (or similar interpreted
503string). Default is a space. (Mnemonic: obvious, I think.)
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504
505=item $SUBSCRIPT_SEPARATOR
506
507=item $SUBSEP
508
509=item $;
a054c801 510X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
a0d0e21e 511
54310121 512The subscript separator for multidimensional array emulation. If you
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513refer to a hash element as
514
515 $foo{$a,$b,$c}
516
517it really means
518
519 $foo{join($;, $a, $b, $c)}
520
521But don't put
522
523 @foo{$a,$b,$c} # a slice--note the @
524
525which means
526
527 ($foo{$a},$foo{$b},$foo{$c})
528
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529Default is "\034", the same as SUBSEP in B<awk>. If your
530keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 531(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 532semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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533taken for something more important.)
534
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535Consider using "real" multidimensional arrays as described
536in L<perllol>.
a0d0e21e 537
fcc7d916 538=item HANDLE->format_page_number(EXPR)
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539
540=item $FORMAT_PAGE_NUMBER
541
542=item $%
a054c801 543X<$%> X<$FORMAT_PAGE_NUMBER>
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544
545The current page number of the currently selected output channel.
19799a22 546Used with formats.
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547(Mnemonic: % is page number in B<nroff>.)
548
fcc7d916 549=item HANDLE->format_lines_per_page(EXPR)
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550
551=item $FORMAT_LINES_PER_PAGE
552
553=item $=
a054c801 554X<$=> X<$FORMAT_LINES_PER_PAGE>
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555
556The current page length (printable lines) of the currently selected
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557output channel. Default is 60.
558Used with formats.
559(Mnemonic: = has horizontal lines.)
a0d0e21e 560
fcc7d916 561=item HANDLE->format_lines_left(EXPR)
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562
563=item $FORMAT_LINES_LEFT
564
565=item $-
a054c801 566X<$-> X<$FORMAT_LINES_LEFT>
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567
568The number of lines left on the page of the currently selected output
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569channel.
570Used with formats.
571(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 572
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573=item @LAST_MATCH_START
574
6cef1e77 575=item @-
a054c801 576X<@-> X<@LAST_MATCH_START>
6cef1e77 577
19799a22 578$-[0] is the offset of the start of the last successful match.
6cef1e77 579C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 580I<n>-th subpattern, or undef if the subpattern did not match.
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581
582Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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583$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
584$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
585C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
14218588
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586matched subgroup in the last successful match. Contrast with
587C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 588with C<@+>.
6cef1e77 589
4ba05bdc
GS
590This array holds the offsets of the beginnings of the last
591successful submatches in the currently active dynamic scope.
592C<$-[0]> is the offset into the string of the beginning of the
593entire match. The I<n>th element of this array holds the offset
0926d669
JP
594of the I<n>th submatch, so C<$-[1]> is the offset where $1
595begins, C<$-[2]> the offset where $2 begins, and so on.
4ba05bdc
GS
596
597After a match against some variable $var:
598
599=over 5
600
4375e838 601=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 602
4375e838 603=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 604
4375e838 605=item C<$'> is the same as C<substr($var, $+[0])>
4ba05bdc
GS
606
607=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
608
609=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
610
80dc6883 611=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
4ba05bdc
GS
612
613=back
614
44a2ac75
YO
615=item %-
616X<%->
617
618Similar to %+, this variable allows access to the named capture
619buffers that were defined in the last successful match. It returns
620a reference to an array containing one value per buffer of a given
621name in the pattern.
622
623 if ('1234'=~/(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
624 foreach my $name (sort keys(%-)) {
625 my $ary = $-{$name};
626 foreach my $idx (0..$#$ary) {
627 print "\$-{$name}[$idx] : ",
628 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
629 "\n";
630 }
631 }
632 }
633
634would print out:
635
636 $-{A}[0] : '1'
637 $-{A}[1] : '3'
638 $-{B}[0] : '2'
639 $-{B}[1] : '4'
640
641The behaviour of %- is implemented via the L<re::Tie::Hash::NamedCapture> module.
642
643Note that C<%-> and C<%+> are tied views into a common internal hash
644associated with the last successful regular expression. Therefore mixing
645iterative access to them via C<each> may have unpredictable results.
646Likewise, if the last successful match changes then the results may be
647surprising.
648
fcc7d916 649=item HANDLE->format_name(EXPR)
a0d0e21e
LW
650
651=item $FORMAT_NAME
652
653=item $~
a054c801 654X<$~> X<$FORMAT_NAME>
a0d0e21e
LW
655
656The name of the current report format for the currently selected output
14218588 657channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 658C<$^>.)
a0d0e21e 659
fcc7d916 660=item HANDLE->format_top_name(EXPR)
a0d0e21e
LW
661
662=item $FORMAT_TOP_NAME
663
664=item $^
a054c801 665X<$^> X<$FORMAT_TOP_NAME>
a0d0e21e
LW
666
667The name of the current top-of-page format for the currently selected
14218588 668output channel. Default is the name of the filehandle with _TOP
a0d0e21e
LW
669appended. (Mnemonic: points to top of page.)
670
46550894 671=item IO::Handle->format_line_break_characters EXPR
a0d0e21e
LW
672
673=item $FORMAT_LINE_BREAK_CHARACTERS
674
675=item $:
a054c801 676X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e
LW
677
678The current set of characters after which a string may be broken to
54310121 679fill continuation fields (starting with ^) in a format. Default is
a0d0e21e
LW
680S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
681poetry is a part of a line.)
682
46550894 683=item IO::Handle->format_formfeed EXPR
a0d0e21e
LW
684
685=item $FORMAT_FORMFEED
686
687=item $^L
a054c801 688X<$^L> X<$FORMAT_FORMFEED>
a0d0e21e 689
14218588 690What formats output as a form feed. Default is \f.
a0d0e21e
LW
691
692=item $ACCUMULATOR
693
694=item $^A
a054c801 695X<$^A> X<$ACCUMULATOR>
a0d0e21e
LW
696
697The current value of the write() accumulator for format() lines. A format
19799a22 698contains formline() calls that put their result into C<$^A>. After
a0d0e21e 699calling its format, write() prints out the contents of C<$^A> and empties.
14218588 700So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
701formline() yourself and then look at it. See L<perlform> and
702L<perlfunc/formline()>.
703
704=item $CHILD_ERROR
705
706=item $?
a054c801 707X<$?> X<$CHILD_ERROR>
a0d0e21e 708
54310121 709The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
710successful call to wait() or waitpid(), or from the system()
711operator. This is just the 16-bit status word returned by the
e5218da5 712traditional Unix wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 713exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
714C<$? & 127> gives which signal, if any, the process died from, and
715C<$? & 128> reports whether there was a core dump. (Mnemonic:
716similar to B<sh> and B<ksh>.)
a0d0e21e 717
7b8d334a 718Additionally, if the C<h_errno> variable is supported in C, its value
14218588 719is returned via $? if any C<gethost*()> function fails.
7b8d334a 720
19799a22 721If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
722value of C<$?> will usually be wrong outside that handler.
723
a8f8344d
PP
724Inside an C<END> subroutine C<$?> contains the value that is going to be
725given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
726change the exit status of your program. For example:
727
728 END {
729 $? = 1 if $? == 255; # die would make it 255
730 }
a8f8344d 731
aa689395 732Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 733actual VMS exit status, instead of the default emulation of POSIX
9bc98430 734status; see L<perlvms/$?> for details.
f86702cc 735
55602bd2
IZ
736Also see L<Error Indicators>.
737
e5218da5 738=item ${^CHILD_ERROR_NATIVE}
a054c801 739X<$^CHILD_ERROR_NATIVE>
e5218da5
GA
740
741The native status returned by the last pipe close, backtick (C<``>)
742command, successful call to wait() or waitpid(), or from the system()
743operator. On POSIX-like systems this value can be decoded with the
744WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
745and WIFCONTINUED functions provided by the L<POSIX> module.
746
747Under VMS this reflects the actual VMS exit status; i.e. it is the same
748as $? when the pragma C<use vmsish 'status'> is in effect.
749
0a378802 750=item ${^ENCODING}
a054c801 751X<$^ENCODING>
0a378802 752
740bd165
PN
753The I<object reference> to the Encode object that is used to convert
754the source code to Unicode. Thanks to this variable your perl script
755does not have to be written in UTF-8. Default is I<undef>. The direct
756manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 757for more details.
0a378802 758
a0d0e21e
LW
759=item $OS_ERROR
760
761=item $ERRNO
762
763=item $!
a054c801 764X<$!> X<$ERRNO> X<$OS_ERROR>
a0d0e21e 765
19799a22 766If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
767variable, or in other words, if a system or library call fails, it
768sets this variable. This means that the value of C<$!> is meaningful
769only I<immediately> after a B<failure>:
770
771 if (open(FH, $filename)) {
772 # Here $! is meaningless.
773 ...
774 } else {
775 # ONLY here is $! meaningful.
776 ...
777 # Already here $! might be meaningless.
778 }
779 # Since here we might have either success or failure,
780 # here $! is meaningless.
781
782In the above I<meaningless> stands for anything: zero, non-zero,
783C<undef>. A successful system or library call does B<not> set
784the variable to zero.
785
271df126 786If used as a string, yields the corresponding system error string.
19799a22
GS
787You can assign a number to C<$!> to set I<errno> if, for instance,
788you want C<"$!"> to return the string for error I<n>, or you want
789to set the exit value for the die() operator. (Mnemonic: What just
790went bang?)
a0d0e21e 791
55602bd2
IZ
792Also see L<Error Indicators>.
793
4c5cef9b 794=item %!
a054c801 795X<%!>
4c5cef9b
MJD
796
797Each element of C<%!> has a true value only if C<$!> is set to that
798value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
799value of C<$!> is C<ENOENT>; that is, if the most recent error was
800"No such file or directory" (or its moral equivalent: not all operating
801systems give that exact error, and certainly not all languages).
802To check if a particular key is meaningful on your system, use
803C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
804See L<Errno> for more information, and also see above for the
805validity of C<$!>.
4c5cef9b 806
5c055ba3
PP
807=item $EXTENDED_OS_ERROR
808
809=item $^E
a054c801 810X<$^E> X<$EXTENDED_OS_ERROR>
5c055ba3 811
22fae026
TM
812Error information specific to the current operating system. At
813the moment, this differs from C<$!> under only VMS, OS/2, and Win32
814(and for MacPerl). On all other platforms, C<$^E> is always just
815the same as C<$!>.
816
817Under VMS, C<$^E> provides the VMS status value from the last
818system error. This is more specific information about the last
819system error than that provided by C<$!>. This is particularly
d516a115 820important when C<$!> is set to B<EVMSERR>.
22fae026 821
1c1c7f20
GS
822Under OS/2, C<$^E> is set to the error code of the last call to
823OS/2 API either via CRT, or directly from perl.
22fae026
TM
824
825Under Win32, C<$^E> always returns the last error information
826reported by the Win32 call C<GetLastError()> which describes
827the last error from within the Win32 API. Most Win32-specific
19799a22 828code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
829set C<errno> and so most portable Perl code will report errors
830via C<$!>.
831
832Caveats mentioned in the description of C<$!> generally apply to
833C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 834
55602bd2
IZ
835Also see L<Error Indicators>.
836
a0d0e21e
LW
837=item $EVAL_ERROR
838
839=item $@
a054c801 840X<$@> X<$EVAL_ERROR>
a0d0e21e 841
4a280ebe
JG
842The Perl syntax error message from the last eval() operator.
843If $@ is the null string, the last eval() parsed and executed
844correctly (although the operations you invoked may have failed in the
845normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 846
19799a22 847Warning messages are not collected in this variable. You can,
a8f8344d 848however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 849as described below.
748a9306 850
55602bd2
IZ
851Also see L<Error Indicators>.
852
a0d0e21e
LW
853=item $PROCESS_ID
854
855=item $PID
856
857=item $$
a054c801 858X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 859
19799a22
GS
860The process number of the Perl running this script. You should
861consider this variable read-only, although it will be altered
862across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 863
4d76a344
RGS
864Note for Linux users: on Linux, the C functions C<getpid()> and
865C<getppid()> return different values from different threads. In order to
866be portable, this behavior is not reflected by C<$$>, whose value remains
867consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 868you may use the CPAN module C<Linux::Pid>.
4d76a344 869
a0d0e21e
LW
870=item $REAL_USER_ID
871
872=item $UID
873
874=item $<
a054c801 875X<< $< >> X<$UID> X<$REAL_USER_ID>
a0d0e21e 876
19799a22 877The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685 878if you're running setuid.) You can change both the real uid and
a537debe
SP
879the effective uid at the same time by using POSIX::setuid(). Since
880changes to $< require a system call, check $! after a change attempt to
881detect any possible errors.
a0d0e21e
LW
882
883=item $EFFECTIVE_USER_ID
884
885=item $EUID
886
887=item $>
a054c801 888X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
a0d0e21e
LW
889
890The effective uid of this process. Example:
891
892 $< = $>; # set real to effective uid
893 ($<,$>) = ($>,$<); # swap real and effective uid
894
a043a685 895You can change both the effective uid and the real uid at the same
a537debe
SP
896time by using POSIX::setuid(). Changes to $> require a check to $!
897to detect any possible errors after an attempted change.
a043a685 898
19799a22 899(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 900C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 901supporting setreuid().
a0d0e21e
LW
902
903=item $REAL_GROUP_ID
904
905=item $GID
906
907=item $(
a054c801 908X<$(> X<$GID> X<$REAL_GROUP_ID>
a0d0e21e
LW
909
910The real gid of this process. If you are on a machine that supports
911membership in multiple groups simultaneously, gives a space separated
912list of groups you are in. The first number is the one returned by
913getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
914the same as the first number.
915
19799a22
GS
916However, a value assigned to C<$(> must be a single number used to
917set the real gid. So the value given by C<$(> should I<not> be assigned
918back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 919
a043a685 920You can change both the real gid and the effective gid at the same
a537debe
SP
921time by using POSIX::setgid(). Changes to $( require a check to $!
922to detect any possible errors after an attempted change.
a043a685 923
19799a22
GS
924(Mnemonic: parentheses are used to I<group> things. The real gid is the
925group you I<left>, if you're running setgid.)
a0d0e21e
LW
926
927=item $EFFECTIVE_GROUP_ID
928
929=item $EGID
930
931=item $)
a054c801 932X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
a0d0e21e
LW
933
934The effective gid of this process. If you are on a machine that
935supports membership in multiple groups simultaneously, gives a space
936separated list of groups you are in. The first number is the one
937returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
938which may be the same as the first number.
939
19799a22 940Similarly, a value assigned to C<$)> must also be a space-separated
14218588 941list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
942the rest (if any) are passed to setgroups(). To get the effect of an
943empty list for setgroups(), just repeat the new effective gid; that is,
944to force an effective gid of 5 and an effectively empty setgroups()
945list, say C< $) = "5 5" >.
946
a043a685
GW
947You can change both the effective gid and the real gid at the same
948time by using POSIX::setgid() (use only a single numeric argument).
a537debe
SP
949Changes to $) require a check to $! to detect any possible errors
950after an attempted change.
a043a685 951
19799a22
GS
952(Mnemonic: parentheses are used to I<group> things. The effective gid
953is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 954
c47ff5f1 955C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
956machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
957and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
958
959=item $PROGRAM_NAME
960
961=item $0
a054c801 962X<$0> X<$PROGRAM_NAME>
a0d0e21e 963
80bca1b4
JH
964Contains the name of the program being executed.
965
966On some (read: not all) operating systems assigning to C<$0> modifies
967the argument area that the C<ps> program sees. On some platforms you
968may have to use special C<ps> options or a different C<ps> to see the
969changes. Modifying the $0 is more useful as a way of indicating the
970current program state than it is for hiding the program you're
971running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277 972
cf525c36 973Note that there are platform specific limitations on the maximum
f9cbb277
JH
974length of C<$0>. In the most extreme case it may be limited to the
975space occupied by the original C<$0>.
a0d0e21e 976
80bca1b4
JH
977In some platforms there may be arbitrary amount of padding, for
978example space characters, after the modified name as shown by C<ps>.
dda345b7 979In some platforms this padding may extend all the way to the original
c80e2480
JH
980length of the argument area, no matter what you do (this is the case
981for example with Linux 2.2).
80bca1b4 982
4bc88a62 983Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3
JH
984from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
985result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
986and the " (perl)" suffix are shown depends on your exact BSD variant
987and version). This is an operating system feature, Perl cannot help it.
4bc88a62 988
e2975953
JH
989In multithreaded scripts Perl coordinates the threads so that any
990thread may modify its copy of the C<$0> and the change becomes visible
cf525c36 991to ps(1) (assuming the operating system plays along). Note that
80bca1b4
JH
992the view of C<$0> the other threads have will not change since they
993have their own copies of it.
e2975953 994
a0d0e21e 995=item $[
a054c801 996X<$[>
a0d0e21e
LW
997
998The index of the first element in an array, and of the first character
19799a22
GS
999in a substring. Default is 0, but you could theoretically set it
1000to 1 to make Perl behave more like B<awk> (or Fortran) when
1001subscripting and when evaluating the index() and substr() functions.
1002(Mnemonic: [ begins subscripts.)
a0d0e21e 1003
19799a22
GS
1004As of release 5 of Perl, assignment to C<$[> is treated as a compiler
1005directive, and cannot influence the behavior of any other file.
f83ed198 1006(That's why you can only assign compile-time constants to it.)
19799a22 1007Its use is highly discouraged.
a0d0e21e 1008
f83ed198 1009Note that, unlike other compile-time directives (such as L<strict>),
af7a0647
RGS
1010assignment to C<$[> can be seen from outer lexical scopes in the same file.
1011However, you can use local() on it to strictly bind its value to a
f83ed198
RGS
1012lexical block.
1013
a0d0e21e 1014=item $]
a054c801 1015X<$]>
a0d0e21e 1016
54310121
PP
1017The version + patchlevel / 1000 of the Perl interpreter. This variable
1018can be used to determine whether the Perl interpreter executing a
1019script is in the right range of versions. (Mnemonic: Is this version
1020of perl in the right bracket?) Example:
a0d0e21e
LW
1021
1022 warn "No checksumming!\n" if $] < 3.019;
1023
54310121 1024See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 1025for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 1026
0c8d858b
MS
1027The floating point representation can sometimes lead to inaccurate
1028numeric comparisons. See C<$^V> for a more modern representation of
1029the Perl version that allows accurate string comparisons.
16070b82 1030
305aace0
NIS
1031=item $COMPILING
1032
1033=item $^C
a054c801 1034X<$^C> X<$COMPILING>
305aace0 1035
19799a22
GS
1036The current value of the flag associated with the B<-c> switch.
1037Mainly of use with B<-MO=...> to allow code to alter its behavior
1038when being compiled, such as for example to AUTOLOAD at compile
1039time rather than normal, deferred loading. See L<perlcc>. Setting
1040C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 1041
a0d0e21e
LW
1042=item $DEBUGGING
1043
1044=item $^D
a054c801 1045X<$^D> X<$DEBUGGING>
a0d0e21e
LW
1046
1047The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c
DM
1048switch.) May be read or set. Like its command-line equivalent, you can use
1049numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e 1050
a3621e74
YO
1051=item ${^RE_DEBUG_FLAGS}
1052
1053The current value of the regex debugging flags. Set to 0 for no debug output
1054even when the re 'debug' module is loaded. See L<re> for details.
1055
0111c4fd 1056=item ${^RE_TRIE_MAXBUF}
a3621e74
YO
1057
1058Controls how certain regex optimisations are applied and how much memory they
1059utilize. This value by default is 65536 which corresponds to a 512kB temporary
1060cache. Set this to a higher value to trade memory for speed when matching
1061large alternations. Set it to a lower value if you want the optimisations to
1062be as conservative of memory as possible but still occur, and set it to a
1063negative value to prevent the optimisation and conserve the most memory.
1064Under normal situations this variable should be of no interest to you.
1065
a0d0e21e
LW
1066=item $SYSTEM_FD_MAX
1067
1068=item $^F
a054c801 1069X<$^F> X<$SYSTEM_FD_MAX>
a0d0e21e
LW
1070
1071The maximum system file descriptor, ordinarily 2. System file
1072descriptors are passed to exec()ed processes, while higher file
1073descriptors are not. Also, during an open(), system file descriptors are
1074preserved even if the open() fails. (Ordinary file descriptors are
19799a22 1075closed before the open() is attempted.) The close-on-exec
a0d0e21e 1076status of a file descriptor will be decided according to the value of
8d2a6795
GS
1077C<$^F> when the corresponding file, pipe, or socket was opened, not the
1078time of the exec().
a0d0e21e 1079
6e2995f4
PP
1080=item $^H
1081
0462a1ab
GS
1082WARNING: This variable is strictly for internal use only. Its availability,
1083behavior, and contents are subject to change without notice.
1084
1085This variable contains compile-time hints for the Perl interpreter. At the
1086end of compilation of a BLOCK the value of this variable is restored to the
1087value when the interpreter started to compile the BLOCK.
1088
1089When perl begins to parse any block construct that provides a lexical scope
1090(e.g., eval body, required file, subroutine body, loop body, or conditional
1091block), the existing value of $^H is saved, but its value is left unchanged.
1092When the compilation of the block is completed, it regains the saved value.
1093Between the points where its value is saved and restored, code that
1094executes within BEGIN blocks is free to change the value of $^H.
1095
1096This behavior provides the semantic of lexical scoping, and is used in,
1097for instance, the C<use strict> pragma.
1098
1099The contents should be an integer; different bits of it are used for
1100different pragmatic flags. Here's an example:
1101
1102 sub add_100 { $^H |= 0x100 }
1103
1104 sub foo {
1105 BEGIN { add_100() }
1106 bar->baz($boon);
1107 }
1108
1109Consider what happens during execution of the BEGIN block. At this point
1110the BEGIN block has already been compiled, but the body of foo() is still
1111being compiled. The new value of $^H will therefore be visible only while
1112the body of foo() is being compiled.
1113
1114Substitution of the above BEGIN block with:
1115
1116 BEGIN { require strict; strict->import('vars') }
1117
1118demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1119version of the same lexical pragma:
1120
1121 BEGIN { require strict; strict->import('vars') if $condition }
1122
1123=item %^H
1124
0462a1ab 1125The %^H hash provides the same scoping semantic as $^H. This makes it
46e5f5f4 1126useful for implementation of lexically scoped pragmas. See L<perlpragma>.
6e2995f4 1127
a0d0e21e
LW
1128=item $INPLACE_EDIT
1129
1130=item $^I
a054c801 1131X<$^I> X<$INPLACE_EDIT>
a0d0e21e
LW
1132
1133The current value of the inplace-edit extension. Use C<undef> to disable
1134inplace editing. (Mnemonic: value of B<-i> switch.)
1135
fb73857a 1136=item $^M
a054c801 1137X<$^M>
fb73857a 1138
19799a22
GS
1139By default, running out of memory is an untrappable, fatal error.
1140However, if suitably built, Perl can use the contents of C<$^M>
1141as an emergency memory pool after die()ing. Suppose that your Perl
0acca065 1142were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
19799a22 1143Then
fb73857a 1144
19799a22 1145 $^M = 'a' x (1 << 16);
fb73857a 1146
51ee6500 1147would allocate a 64K buffer for use in an emergency. See the
19799a22 1148F<INSTALL> file in the Perl distribution for information on how to
0acca065
RGS
1149add custom C compilation flags when compiling perl. To discourage casual
1150use of this advanced feature, there is no L<English|English> long name for
1151this variable.
fb73857a 1152
5c055ba3 1153=item $OSNAME
6e2995f4 1154
5c055ba3 1155=item $^O
a054c801 1156X<$^O> X<$OSNAME>
5c055ba3
PP
1157
1158The name of the operating system under which this copy of Perl was
1159built, as determined during the configuration process. The value
19799a22
GS
1160is identical to C<$Config{'osname'}>. See also L<Config> and the
1161B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1162
443f6d01 1163In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1164C<MSWin32>, it doesn't tell the difference between
116595/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1166Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1167between the variants.
916d64a3 1168
e2e27056
JH
1169=item ${^OPEN}
1170
1171An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1172by a C<\0> byte, the first part describes the input layers, the second
1173part describes the output layers.
e2e27056 1174
a0d0e21e
LW
1175=item $PERLDB
1176
1177=item $^P
a054c801 1178X<$^P> X<$PERLDB>
a0d0e21e 1179
19799a22
GS
1180The internal variable for debugging support. The meanings of the
1181various bits are subject to change, but currently indicate:
84902520
TB
1182
1183=over 6
1184
1185=item 0x01
1186
1187Debug subroutine enter/exit.
1188
1189=item 0x02
1190
1191Line-by-line debugging.
1192
1193=item 0x04
1194
1195Switch off optimizations.
1196
1197=item 0x08
1198
1199Preserve more data for future interactive inspections.
1200
1201=item 0x10
1202
1203Keep info about source lines on which a subroutine is defined.
1204
1205=item 0x20
1206
1207Start with single-step on.
1208
83ee9e09
GS
1209=item 0x40
1210
1211Use subroutine address instead of name when reporting.
1212
1213=item 0x80
1214
1215Report C<goto &subroutine> as well.
1216
1217=item 0x100
1218
1219Provide informative "file" names for evals based on the place they were compiled.
1220
1221=item 0x200
1222
1223Provide informative names to anonymous subroutines based on the place they
1224were compiled.
1225
7619c85e
RG
1226=item 0x400
1227
1228Debug assertion subroutines enter/exit.
1229
84902520
TB
1230=back
1231
19799a22
GS
1232Some bits may be relevant at compile-time only, some at
1233run-time only. This is a new mechanism and the details may change.
a0d0e21e 1234
66558a10
GS
1235=item $LAST_REGEXP_CODE_RESULT
1236
b9ac3b5b 1237=item $^R
a054c801 1238X<$^R> X<$LAST_REGEXP_CODE_RESULT>
b9ac3b5b 1239
19799a22
GS
1240The result of evaluation of the last successful C<(?{ code })>
1241regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1242
66558a10
GS
1243=item $EXCEPTIONS_BEING_CAUGHT
1244
fb73857a 1245=item $^S
a054c801 1246X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
fb73857a 1247
fa05a9fd
IST
1248Current state of the interpreter.
1249
1250 $^S State
1251 --------- -------------------
1252 undef Parsing module/eval
1253 true (1) Executing an eval
1254 false (0) Otherwise
1255
1256The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1257
a0d0e21e
LW
1258=item $BASETIME
1259
1260=item $^T
a054c801 1261X<$^T> X<$BASETIME>
a0d0e21e 1262
19799a22 1263The time at which the program began running, in seconds since the
5f05dabc 1264epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1265and B<-C> filetests are based on this value.
a0d0e21e 1266
7c36658b
MS
1267=item ${^TAINT}
1268
9aa05f58
RGS
1269Reflects if taint mode is on or off. 1 for on (the program was run with
1270B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1271B<-t> or B<-TU>). This variable is read-only.
7c36658b 1272
a05d7ebb
JH
1273=item ${^UNICODE}
1274
ab9e1bb7
JH
1275Reflects certain Unicode settings of Perl. See L<perlrun>
1276documentation for the C<-C> switch for more information about
1277the possible values. This variable is set during Perl startup
1278and is thereafter read-only.
fde18df1 1279
e07ea26a
NC
1280=item ${^UTF8CACHE}
1281
1282This variable controls the state of the internal UTF-8 offset caching code.
16d9fe92
NC
12831 for on (the default), 0 for off, -1 to debug the caching code by checking
1284all its results against linear scans, and panicking on any discrepancy.
e07ea26a 1285
ea8eae40
RGS
1286=item ${^UTF8LOCALE}
1287
1288This variable indicates whether an UTF-8 locale was detected by perl at
1289startup. This information is used by perl when it's in
1290adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1291switch); see L<perlrun> for more info on this.
1292
44dcb63b 1293=item $PERL_VERSION
b459063d 1294
16070b82 1295=item $^V
a054c801 1296X<$^V> X<$PERL_VERSION>
16070b82
GS
1297
1298The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1299as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1300it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1301C<$^V eq v5.6.0>. Note that the characters in this string value can
1302potentially be in Unicode range.
16070b82 1303
7d2b1222
DM
1304This variable first appeared in perl 5.6.0; earlier versions of perl will
1305see an undefined value.
1306
16070b82
GS
1307This can be used to determine whether the Perl interpreter executing a
1308script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1309Control.) Example:
16070b82 1310
7d2b1222 1311 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
16070b82 1312
aa2f2a36
AMS
1313To convert C<$^V> into its string representation use sprintf()'s
1314C<"%vd"> conversion:
1315
1316 printf "version is v%vd\n", $^V; # Perl's version
1317
44dcb63b 1318See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1319for a convenient way to fail if the running Perl interpreter is too old.
1320
1321See also C<$]> for an older representation of the Perl version.
1322
a0d0e21e
LW
1323=item $WARNING
1324
1325=item $^W
a054c801 1326X<$^W> X<$WARNING>
a0d0e21e 1327
19799a22
GS
1328The current value of the warning switch, initially true if B<-w>
1329was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1330related to the B<-w> switch.) See also L<warnings>.
1331
6a818117 1332=item ${^WARNING_BITS}
4438c4b7
JH
1333
1334The current set of warning checks enabled by the C<use warnings> pragma.
1335See the documentation of C<warnings> for more details.
a0d0e21e 1336
2a8c8378
JD
1337=item ${^WIN32_SLOPPY_STAT}
1338
1339If this variable is set to a true value, then stat() on Windows will
1340not try to open the file. This means that the link count cannot be
1341determined and file attributes may be out of date if additional
1342hardlinks to the file exist. On the other hand, not opening the file
1343is considerably faster, especially for files on network drives.
1344
1345This variable could be set in the F<sitecustomize.pl> file to
1346configure the local Perl installation to use "sloppy" stat() by
1347default. See L<perlrun> for more information about site
1348customization.
1349
a0d0e21e
LW
1350=item $EXECUTABLE_NAME
1351
1352=item $^X
a054c801 1353X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 1354
e71940de 1355The name used to execute the current copy of Perl, from C's
21c1191d 1356C<argv[0]> or (where supported) F</proc/self/exe>.
38e4f4ae 1357
e71940de
PG
1358Depending on the host operating system, the value of $^X may be
1359a relative or absolute pathname of the perl program file, or may
1360be the string used to invoke perl but not the pathname of the
1361perl program file. Also, most operating systems permit invoking
1362programs that are not in the PATH environment variable, so there
a10d74f3
PG
1363is no guarantee that the value of $^X is in PATH. For VMS, the
1364value may or may not include a version number.
38e4f4ae 1365
e71940de
PG
1366You usually can use the value of $^X to re-invoke an independent
1367copy of the same perl that is currently running, e.g.,
1368
1369 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1370
1371But recall that not all operating systems support forking or
1372capturing of the output of commands, so this complex statement
1373may not be portable.
38e4f4ae 1374
e71940de
PG
1375It is not safe to use the value of $^X as a path name of a file,
1376as some operating systems that have a mandatory suffix on
1377executable files do not require use of the suffix when invoking
1378a command. To convert the value of $^X to a path name, use the
1379following statements:
1380
304dea91 1381 # Build up a set of file names (not command names).
e71940de 1382 use Config;
68fb0eb7
PG
1383 $this_perl = $^X;
1384 if ($^O ne 'VMS')
1385 {$this_perl .= $Config{_exe}
1386 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1387
1388Because many operating systems permit anyone with read access to
1389the Perl program file to make a copy of it, patch the copy, and
1390then execute the copy, the security-conscious Perl programmer
1391should take care to invoke the installed copy of perl, not the
1392copy referenced by $^X. The following statements accomplish
1393this goal, and produce a pathname that can be invoked as a
1394command or referenced as a file.
38e4f4ae
SB
1395
1396 use Config;
68fb0eb7
PG
1397 $secure_perl_path = $Config{perlpath};
1398 if ($^O ne 'VMS')
1399 {$secure_perl_path .= $Config{_exe}
1400 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1401
2d84a16a 1402=item ARGV
a054c801 1403X<ARGV>
2d84a16a
DM
1404
1405The special filehandle that iterates over command-line filenames in
1406C<@ARGV>. Usually written as the null filehandle in the angle operator
1407C<< <> >>. Note that currently C<ARGV> only has its magical effect
1408within the C<< <> >> operator; elsewhere it is just a plain filehandle
1409corresponding to the last file opened by C<< <> >>. In particular,
1410passing C<\*ARGV> as a parameter to a function that expects a filehandle
1411may not cause your function to automatically read the contents of all the
1412files in C<@ARGV>.
1413
a0d0e21e 1414=item $ARGV
a054c801 1415X<$ARGV>
a0d0e21e 1416
c47ff5f1 1417contains the name of the current file when reading from <>.
a0d0e21e
LW
1418
1419=item @ARGV
a054c801 1420X<@ARGV>
a0d0e21e 1421
19799a22 1422The array @ARGV contains the command-line arguments intended for
14218588 1423the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1424one, because C<$ARGV[0]> is the first argument, I<not> the program's
1425command name itself. See C<$0> for the command name.
a0d0e21e 1426
5ccee41e 1427=item ARGVOUT
a054c801 1428X<ARGVOUT>
5ccee41e
JA
1429
1430The special filehandle that points to the currently open output file
1431when doing edit-in-place processing with B<-i>. Useful when you have
1432to do a lot of inserting and don't want to keep modifying $_. See
1433L<perlrun> for the B<-i> switch.
1434
9b0e6e7a 1435=item @F
a054c801 1436X<@F>
9b0e6e7a
JP
1437
1438The array @F contains the fields of each line read in when autosplit
1439mode is turned on. See L<perlrun> for the B<-a> switch. This array
1440is package-specific, and must be declared or given a full package name
1441if not in package main when running under C<strict 'vars'>.
1442
a0d0e21e 1443=item @INC
a054c801 1444X<@INC>
a0d0e21e 1445
19799a22
GS
1446The array @INC contains the list of places that the C<do EXPR>,
1447C<require>, or C<use> constructs look for their library files. It
1448initially consists of the arguments to any B<-I> command-line
1449switches, followed by the default Perl library, probably
1450F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1451directory. ("." will not be appended if taint checks are enabled, either by
1452C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1453the C<use lib> pragma to get the machine-dependent library properly
1454loaded also:
a0d0e21e 1455
cb1a09d0
AD
1456 use lib '/mypath/libdir/';
1457 use SomeMod;
303f2f76 1458
d54b56d5
RGS
1459You can also insert hooks into the file inclusion system by putting Perl
1460code directly into @INC. Those hooks may be subroutine references, array
1461references or blessed objects. See L<perlfunc/require> for details.
1462
314d39ce
MG
1463=item @ARG
1464
fb73857a 1465=item @_
a054c801 1466X<@_> X<@ARG>
fb73857a
PP
1467
1468Within a subroutine the array @_ contains the parameters passed to that
19799a22 1469subroutine. See L<perlsub>.
fb73857a 1470
a0d0e21e 1471=item %INC
a054c801 1472X<%INC>
a0d0e21e 1473
19799a22
GS
1474The hash %INC contains entries for each filename included via the
1475C<do>, C<require>, or C<use> operators. The key is the filename
1476you specified (with module names converted to pathnames), and the
14218588 1477value is the location of the file found. The C<require>
87275199 1478operator uses this hash to determine whether a particular file has
19799a22 1479already been included.
a0d0e21e 1480
89ccab8c
RGS
1481If the file was loaded via a hook (e.g. a subroutine reference, see
1482L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1483by default inserted into %INC in place of a filename. Note, however,
1484that the hook may have set the %INC entry by itself to provide some more
1485specific info.
44f0be63 1486
b687b08b
TC
1487=item %ENV
1488
1489=item $ENV{expr}
a054c801 1490X<%ENV>
a0d0e21e
LW
1491
1492The hash %ENV contains your current environment. Setting a
19799a22
GS
1493value in C<ENV> changes the environment for any child processes
1494you subsequently fork() off.
a0d0e21e 1495
b687b08b
TC
1496=item %SIG
1497
1498=item $SIG{expr}
a054c801 1499X<%SIG>
a0d0e21e 1500
efbd929d 1501The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e
LW
1502
1503 sub handler { # 1st argument is signal name
fb73857a 1504 my($sig) = @_;
a0d0e21e
LW
1505 print "Caught a SIG$sig--shutting down\n";
1506 close(LOG);
1507 exit(0);
1508 }
1509
fb73857a
PP
1510 $SIG{'INT'} = \&handler;
1511 $SIG{'QUIT'} = \&handler;
a0d0e21e 1512 ...
19799a22 1513 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1514 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1515
f648820c
GS
1516Using a value of C<'IGNORE'> usually has the effect of ignoring the
1517signal, except for the C<CHLD> signal. See L<perlipc> for more about
1518this special case.
1519
19799a22 1520Here are some other examples:
a0d0e21e 1521
fb73857a 1522 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1523 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1524 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1525 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1526
19799a22
GS
1527Be sure not to use a bareword as the name of a signal handler,
1528lest you inadvertently call it.
748a9306 1529
44a8e56a 1530If your system has the sigaction() function then signal handlers are
9ce5b4ad 1531installed using it. This means you get reliable signal handling.
44a8e56a 1532
9ce5b4ad
SG
1533The default delivery policy of signals changed in Perl 5.8.0 from
1534immediate (also known as "unsafe") to deferred, also known as
1535"safe signals". See L<perlipc> for more information.
45c0772f 1536
748a9306 1537Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1538routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306 1539about to be printed. The warning message is passed as the first
efbd929d
AT
1540argument. The presence of a C<__WARN__> hook causes the ordinary printing
1541of warnings to C<STDERR> to be suppressed. You can use this to save warnings
748a9306
LW
1542in a variable, or turn warnings into fatal errors, like this:
1543
1544 local $SIG{__WARN__} = sub { die $_[0] };
1545 eval $proggie;
1546
efbd929d
AT
1547As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
1548disable warnings using the empty subroutine:
1549
1550 local $SIG{__WARN__} = sub {};
1551
a8f8344d 1552The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306 1553is about to be thrown. The error message is passed as the first
efbd929d 1554argument. When a C<__DIE__> hook routine returns, the exception
748a9306 1555processing continues as it would have in the absence of the hook,
efbd929d 1556unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>.
774d564b 1557The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1558can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1559
19799a22
GS
1560Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1561even inside an eval(). Do not use this to rewrite a pending exception
efbd929d 1562in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>.
19799a22
GS
1563This strange action at a distance may be fixed in a future release
1564so that C<$SIG{__DIE__}> is only called if your program is about
1565to exit, as was the original intent. Any other use is deprecated.
1566
1567C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1568they may be called to report (probable) errors found by the parser.
1569In such a case the parser may be in inconsistent state, so any
1570attempt to evaluate Perl code from such a handler will probably
1571result in a segfault. This means that warnings or errors that
1572result from parsing Perl should be used with extreme caution, like
1573this:
fb73857a
PP
1574
1575 require Carp if defined $^S;
1576 Carp::confess("Something wrong") if defined &Carp::confess;
1577 die "Something wrong, but could not load Carp to give backtrace...
1578 To see backtrace try starting Perl with -MCarp switch";
1579
1580Here the first line will load Carp I<unless> it is the parser who
1581called the handler. The second line will print backtrace and die if
1582Carp was available. The third line will be executed only if Carp was
1583not available.
1584
19799a22 1585See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1586L<warnings> for additional information.
68dc0745 1587
a0d0e21e 1588=back
55602bd2
IZ
1589
1590=head2 Error Indicators
a054c801 1591X<error> X<exception>
55602bd2 1592
19799a22
GS
1593The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1594about different types of error conditions that may appear during
1595execution of a Perl program. The variables are shown ordered by
1596the "distance" between the subsystem which reported the error and
1597the Perl process. They correspond to errors detected by the Perl
1598interpreter, C library, operating system, or an external program,
1599respectively.
55602bd2
IZ
1600
1601To illustrate the differences between these variables, consider the
19799a22 1602following Perl expression, which uses a single-quoted string:
55602bd2 1603
19799a22 1604 eval q{
22d0716c
SB
1605 open my $pipe, "/cdrom/install |" or die $!;
1606 my @res = <$pipe>;
1607 close $pipe or die "bad pipe: $?, $!";
19799a22 1608 };
55602bd2
IZ
1609
1610After execution of this statement all 4 variables may have been set.
1611
19799a22
GS
1612C<$@> is set if the string to be C<eval>-ed did not compile (this
1613may happen if C<open> or C<close> were imported with bad prototypes),
1614or if Perl code executed during evaluation die()d . In these cases
1615the value of $@ is the compile error, or the argument to C<die>
4cb1c523 1616(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
19799a22
GS
1617though.)
1618
c47ff5f1 1619When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1620and C<close> are translated to calls in the C run-time library and
1621thence to the operating system kernel. C<$!> is set to the C library's
1622C<errno> if one of these calls fails.
1623
1624Under a few operating systems, C<$^E> may contain a more verbose
1625error indicator, such as in this case, "CDROM tray not closed."
14218588 1626Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1627the same as C<$!>.
1628
1629Finally, C<$?> may be set to non-0 value if the external program
1630F</cdrom/install> fails. The upper eight bits reflect specific
1631error conditions encountered by the program (the program's exit()
1632value). The lower eight bits reflect mode of failure, like signal
1633death and core dump information See wait(2) for details. In
1634contrast to C<$!> and C<$^E>, which are set only if error condition
1635is detected, the variable C<$?> is set on each C<wait> or pipe
1636C<close>, overwriting the old value. This is more like C<$@>, which
1637on every eval() is always set on failure and cleared on success.
2b92dfce 1638
19799a22
GS
1639For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1640and C<$?>.
2b92dfce
GS
1641
1642=head2 Technical Note on the Syntax of Variable Names
1643
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GS
1644Variable names in Perl can have several formats. Usually, they
1645must begin with a letter or underscore, in which case they can be
1646arbitrarily long (up to an internal limit of 251 characters) and
1647may contain letters, digits, underscores, or the special sequence
1648C<::> or C<'>. In this case, the part before the last C<::> or
1649C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1650
1651Perl variable names may also be a sequence of digits or a single
1652punctuation or control character. These names are all reserved for
19799a22
GS
1653special uses by Perl; for example, the all-digits names are used
1654to hold data captured by backreferences after a regular expression
1655match. Perl has a special syntax for the single-control-character
1656names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1657character. For example, the notation C<$^W> (dollar-sign caret
1658C<W>) is the scalar variable whose name is the single character
1659control-C<W>. This is better than typing a literal control-C<W>
1660into your program.
2b92dfce 1661
87275199 1662Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1663strings that begin with control characters (or better yet, a caret).
1664These variables must be written in the form C<${^Foo}>; the braces
1665are not optional. C<${^Foo}> denotes the scalar variable whose
1666name is a control-C<F> followed by two C<o>'s. These variables are
1667reserved for future special uses by Perl, except for the ones that
1668begin with C<^_> (control-underscore or caret-underscore). No
1669control-character name that begins with C<^_> will acquire a special
1670meaning in any future version of Perl; such names may therefore be
1671used safely in programs. C<$^_> itself, however, I<is> reserved.
1672
1fcb18de
RGS
1673Perl identifiers that begin with digits, control characters, or
1674punctuation characters are exempt from the effects of the C<package>
1675declaration and are always forced to be in package C<main>; they are
1676also exempt from C<strict 'vars'> errors. A few other names are also
1677exempt in these ways:
2b92dfce
GS
1678
1679 ENV STDIN
1680 INC STDOUT
1681 ARGV STDERR
5b88253b 1682 ARGVOUT _
2b92dfce
GS
1683 SIG
1684
1685In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1686to be in package C<main>, regardless of any C<package> declarations
747fafda 1687presently in scope.
2b92dfce 1688
19799a22
GS
1689=head1 BUGS
1690
1691Due to an unfortunate accident of Perl's implementation, C<use
1692English> imposes a considerable performance penalty on all regular
1693expression matches in a program, regardless of whether they occur
1694in the scope of C<use English>. For that reason, saying C<use
1695English> in libraries is strongly discouraged. See the
1696Devel::SawAmpersand module documentation from CPAN
1577cd80 1697( http://www.cpan.org/modules/by-module/Devel/ )
a054c801
GS
1698for more information. Writing C<use English '-no_match_vars';>
1699avoids the performance penalty.
2b92dfce 1700
19799a22
GS
1701Having to even think about the C<$^S> variable in your exception
1702handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1703invites grievous and difficult to track down errors. Avoid it
1704and use an C<END{}> or CORE::GLOBAL::die override instead.