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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
b0169937 61 open my $fh, "<", "foo" or die $!;
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62 local $/; # enable localized slurp mode
63 my $content = <$fh>;
64 close $fh;
65
66But the following code is quite bad:
67
b0169937 68 open my $fh, "<", "foo" or die $!;
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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 = '';
b0169937 84 open my $fh, "<", "foo" or die $!;
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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 $_ even if you
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145don't use it:
146
147=over 3
148
149=item *
150
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151The following functions:
152
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153abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
154hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
155quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
156rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
157unlink, unpack.
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158
159=item *
160
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161All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
162See L<perlfunc/-X>
163
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164
165=item *
166
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167The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
168when used without an C<=~> operator.
cb1a09d0 169
54310121 170=item *
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171
172The default iterator variable in a C<foreach> loop if no other
173variable is supplied.
174
54310121 175=item *
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176
177The implicit iterator variable in the grep() and map() functions.
178
54310121 179=item *
cb1a09d0 180
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181The implicit variable of given().
182
183=item *
184
c47ff5f1 185The default place to put an input record when a C<< <FH> >>
cb1a09d0 186operation's result is tested by itself as the sole criterion of a C<while>
14218588 187test. Outside a C<while> test, this will not happen.
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188
189=back
190
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191As C<$_> is a global variable, this may lead in some cases to unwanted
192side-effects. As of perl 5.9.1, you can now use a lexical version of
193C<$_> by declaring it in a file or in a block with C<my>. Moreover,
4fd88bf8 194declaring C<our $_> restores the global C<$_> in the current scope.
59f00321 195
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196(Mnemonic: underline is understood in certain operations.)
197
6e2995f4 198=back
199
200=over 8
201
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202=item $a
203
204=item $b
a054c801 205X<$a> X<$b>
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206
207Special package variables when using sort(), see L<perlfunc/sort>.
208Because of this specialness $a and $b don't need to be declared
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209(using use vars, or our()) even when using the C<strict 'vars'> pragma.
210Don't lexicalize them with C<my $a> or C<my $b> if you want to be
211able to use them in the sort() comparison block or function.
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212
213=back
214
215=over 8
216
8491f48b 217=item $<I<digits>> ($1, $2, ...)
a054c801 218X<$1> X<$2> X<$3>
a0d0e21e 219
19799a22 220Contains the subpattern from the corresponding set of capturing
8491f48b 221parentheses from the last successful pattern match, not counting patterns
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222matched in nested blocks that have been exited already. (Mnemonic:
223like \digits.) These variables are all read-only and dynamically
224scoped to the current BLOCK.
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225
226=item $MATCH
227
228=item $&
a054c801 229X<$&> X<$MATCH>
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230
231The string matched by the last successful pattern match (not counting
232any matches hidden within a BLOCK or eval() enclosed by the current
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233BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
234and dynamically scoped to the current BLOCK.
a0d0e21e 235
19ddd453 236The use of this variable anywhere in a program imposes a considerable
667e1aea 237performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 238
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239See L</@-> for a replacement.
240
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241=item ${^MATCH}
242X<${^MATCH}>
243
6210e03f 244This is similar to C<$&> (C<$MATCH>) except that it does not incur the
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245performance penalty associated with that variable, and is only guaranteed
246to return a defined value when the pattern was compiled or executed with
87e95b7f 247the C</p> modifier.
cde0cee5 248
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249=item $PREMATCH
250
251=item $`
a054c801 252X<$`> X<$PREMATCH>
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253
254The string preceding whatever was matched by the last successful
255pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 256enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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257string.) This variable is read-only.
258
19ddd453 259The use of this variable anywhere in a program imposes a considerable
667e1aea 260performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 261
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262See L</@-> for a replacement.
263
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264=item ${^PREMATCH}
265X<${^PREMATCH}>
266
267This is similar to C<$`> ($PREMATCH) except that it does not incur the
268performance penalty associated with that variable, and is only guaranteed
269to return a defined value when the pattern was compiled or executed with
87e95b7f 270the C</p> modifier.
cde0cee5 271
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272=item $POSTMATCH
273
274=item $'
a054c801 275X<$'> X<$POSTMATCH>
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276
277The string following whatever was matched by the last successful
278pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 279enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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280string.) Example:
281
22d0716c 282 local $_ = 'abcdefghi';
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283 /def/;
284 print "$`:$&:$'\n"; # prints abc:def:ghi
285
19799a22 286This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 287
19ddd453 288The use of this variable anywhere in a program imposes a considerable
667e1aea 289performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 290
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291See L</@-> for a replacement.
292
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293=item ${^POSTMATCH}
294X<${^POSTMATCH}>
295
296This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
297performance penalty associated with that variable, and is only guaranteed
298to return a defined value when the pattern was compiled or executed with
87e95b7f 299the C</p> modifier.
cde0cee5 300
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301=item $LAST_PAREN_MATCH
302
303=item $+
a054c801 304X<$+> X<$LAST_PAREN_MATCH>
a0d0e21e 305
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306The text matched by the last bracket of the last successful search pattern.
307This is useful if you don't know which one of a set of alternative patterns
308matched. For example:
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309
310 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
311
312(Mnemonic: be positive and forward looking.)
19799a22 313This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 314
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315=item $LAST_SUBMATCH_RESULT
316
a01268b5 317=item $^N
a054c801 318X<$^N>
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319
320The text matched by the used group most-recently closed (i.e. the group
321with the rightmost closing parenthesis) of the last successful search
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322pattern. (Mnemonic: the (possibly) Nested parenthesis that most
323recently closed.)
324
210b36aa 325This is primarily used inside C<(?{...})> blocks for examining text
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326recently matched. For example, to effectively capture text to a variable
327(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
328
329 (?:(...)(?{ $var = $^N }))
330
331By setting and then using C<$var> in this way relieves you from having to
332worry about exactly which numbered set of parentheses they are.
333
334This variable is dynamically scoped to the current BLOCK.
335
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336=item @LAST_MATCH_END
337
6cef1e77 338=item @+
a054c801 339X<@+> X<@LAST_MATCH_END>
6cef1e77 340
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341This array holds the offsets of the ends of the last successful
342submatches in the currently active dynamic scope. C<$+[0]> is
343the offset into the string of the end of the entire match. This
344is the same value as what the C<pos> function returns when called
345on the variable that was matched against. The I<n>th element
346of this array holds the offset of the I<n>th submatch, so
347C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
348past where $2 ends, and so on. You can use C<$#+> to determine
349how many subgroups were in the last successful match. See the
350examples given for the C<@-> variable.
6cef1e77 351
43239f90 352=item %LAST_PAREN_MATCH
8e08999f 353
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354=item %+
355X<%+>
356
357Similar to C<@+>, the C<%+> hash allows access to the named capture
358buffers, should they exist, in the last successful match in the
359currently active dynamic scope.
360
3195cf34 361For example, C<$+{foo}> is equivalent to C<$1> after the following match:
81714fb9 362
3195cf34 363 'foo' =~ /(?<foo>foo)/;
81714fb9 364
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365The keys of the C<%+> hash list only the names of buffers that have
366captured (and that are thus associated to defined values).
44a2ac75 367
3195cf34 368The underlying behaviour of C<%+> is provided by the
80305961 369L<Tie::Hash::NamedCapture> module.
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370
371B<Note:> C<%-> and C<%+> are tied views into a common internal hash
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372associated with the last successful regular expression. Therefore mixing
373iterative access to them via C<each> may have unpredictable results.
3195cf34 374Likewise, if the last successful match changes, then the results may be
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375surprising.
376
fcc7d916 377=item HANDLE->input_line_number(EXPR)
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378
379=item $INPUT_LINE_NUMBER
380
381=item $NR
382
383=item $.
a054c801 384X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
a0d0e21e 385
81714fb9 386Current line number for the last filehandle accessed.
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387
388Each filehandle in Perl counts the number of lines that have been read
389from it. (Depending on the value of C<$/>, Perl's idea of what
390constitutes a line may not match yours.) When a line is read from a
391filehandle (via readline() or C<< <> >>), or when tell() or seek() is
392called on it, C<$.> becomes an alias to the line counter for that
393filehandle.
394
395You can adjust the counter by assigning to C<$.>, but this will not
396actually move the seek pointer. I<Localizing C<$.> will not localize
397the filehandle's line count>. Instead, it will localize perl's notion
398of which filehandle C<$.> is currently aliased to.
399
400C<$.> is reset when the filehandle is closed, but B<not> when an open
401filehandle is reopened without an intervening close(). For more
e48df184 402details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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403an explicit close, line numbers increase across ARGV files (but see
404examples in L<perlfunc/eof>).
405
406You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
407line counter for a given filehandle without having to worry about
408which handle you last accessed.
409
410(Mnemonic: many programs use "." to mean the current line number.)
411
412=item IO::Handle->input_record_separator(EXPR)
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413
414=item $INPUT_RECORD_SEPARATOR
415
416=item $RS
417
418=item $/
a054c801 419X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
a0d0e21e 420
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421The input record separator, newline by default. This
422influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 423variable, including treating empty lines as a terminator if set to
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424the null string. (An empty line cannot contain any spaces
425or tabs.) You may set it to a multi-character string to match a
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426multi-character terminator, or to C<undef> to read through the end
427of file. Setting it to C<"\n\n"> means something slightly
428different than setting to C<"">, if the file contains consecutive
429empty lines. Setting to C<""> will treat two or more consecutive
430empty lines as a single empty line. Setting to C<"\n\n"> will
431blindly assume that the next input character belongs to the next
14218588 432paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 433line boundaries when quoting poetry.)
a0d0e21e 434
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435 local $/; # enable "slurp" mode
436 local $_ = <FH>; # whole file now here
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437 s/\n[ \t]+/ /g;
438
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439Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
440better for something. :-)
68dc0745 441
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442Setting C<$/> to a reference to an integer, scalar containing an integer, or
443scalar that's convertible to an integer will attempt to read records
5b2b9c68 444instead of lines, with the maximum record size being the referenced
19799a22 445integer. So this:
5b2b9c68 446
22d0716c 447 local $/ = \32768; # or \"32768", or \$var_containing_32768
b0169937 448 open my $fh, "<", $myfile or die $!;
22d0716c 449 local $_ = <$fh>;
5b2b9c68 450
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451will read a record of no more than 32768 bytes from FILE. If you're
452not reading from a record-oriented file (or your OS doesn't have
453record-oriented files), then you'll likely get a full chunk of data
454with every read. If a record is larger than the record size you've
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455set, you'll get the record back in pieces. Trying to set the record
456size to zero or less will cause reading in the (rest of the) whole file.
5b2b9c68 457
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458On VMS, record reads are done with the equivalent of C<sysread>,
459so it's best not to mix record and non-record reads on the same
460file. (This is unlikely to be a problem, because any file you'd
83763826 461want to read in record mode is probably unusable in line mode.)
14218588 462Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 463non-record reads of a file.
5b2b9c68 464
14218588 465See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 466
fcc7d916 467=item HANDLE->autoflush(EXPR)
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468
469=item $OUTPUT_AUTOFLUSH
470
471=item $|
a054c801 472X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
a0d0e21e 473
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474If set to nonzero, forces a flush right away and after every write
475or print on the currently selected output channel. Default is 0
14218588 476(regardless of whether the channel is really buffered by the
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477system or not; C<$|> tells you only whether you've asked Perl
478explicitly to flush after each write). STDOUT will
479typically be line buffered if output is to the terminal and block
480buffered otherwise. Setting this variable is useful primarily when
481you are outputting to a pipe or socket, such as when you are running
482a Perl program under B<rsh> and want to see the output as it's
483happening. This has no effect on input buffering. See L<perlfunc/getc>
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484for that. See L<perldoc/select> on how to select the output channel.
485See also L<IO::Handle>. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 486
46550894 487=item IO::Handle->output_field_separator EXPR
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488
489=item $OUTPUT_FIELD_SEPARATOR
490
491=item $OFS
492
493=item $,
a054c801 494X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
a0d0e21e 495
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496The output field separator for the print operator. If defined, this
497value is printed between each of print's arguments. Default is C<undef>.
498(Mnemonic: what is printed when there is a "," in your print statement.)
a0d0e21e 499
46550894 500=item IO::Handle->output_record_separator EXPR
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501
502=item $OUTPUT_RECORD_SEPARATOR
503
504=item $ORS
505
506=item $\
a054c801 507X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
a0d0e21e 508
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509The output record separator for the print operator. If defined, this
510value is printed after the last of print's arguments. Default is C<undef>.
511(Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
512Also, it's just like C<$/>, but it's what you get "back" from Perl.)
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513
514=item $LIST_SEPARATOR
515
516=item $"
a054c801 517X<$"> X<$LIST_SEPARATOR>
a0d0e21e 518
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519This is like C<$,> except that it applies to array and slice values
520interpolated into a double-quoted string (or similar interpreted
521string). Default is a space. (Mnemonic: obvious, I think.)
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522
523=item $SUBSCRIPT_SEPARATOR
524
525=item $SUBSEP
526
527=item $;
a054c801 528X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
a0d0e21e 529
54310121 530The subscript separator for multidimensional array emulation. If you
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531refer to a hash element as
532
533 $foo{$a,$b,$c}
534
535it really means
536
537 $foo{join($;, $a, $b, $c)}
538
539But don't put
540
541 @foo{$a,$b,$c} # a slice--note the @
542
543which means
544
545 ($foo{$a},$foo{$b},$foo{$c})
546
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547Default is "\034", the same as SUBSEP in B<awk>. If your
548keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 549(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 550semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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551taken for something more important.)
552
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553Consider using "real" multidimensional arrays as described
554in L<perllol>.
a0d0e21e 555
fcc7d916 556=item HANDLE->format_page_number(EXPR)
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557
558=item $FORMAT_PAGE_NUMBER
559
560=item $%
a054c801 561X<$%> X<$FORMAT_PAGE_NUMBER>
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562
563The current page number of the currently selected output channel.
19799a22 564Used with formats.
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565(Mnemonic: % is page number in B<nroff>.)
566
fcc7d916 567=item HANDLE->format_lines_per_page(EXPR)
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568
569=item $FORMAT_LINES_PER_PAGE
570
571=item $=
a054c801 572X<$=> X<$FORMAT_LINES_PER_PAGE>
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573
574The current page length (printable lines) of the currently selected
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575output channel. Default is 60.
576Used with formats.
577(Mnemonic: = has horizontal lines.)
a0d0e21e 578
fcc7d916 579=item HANDLE->format_lines_left(EXPR)
a0d0e21e
LW
580
581=item $FORMAT_LINES_LEFT
582
583=item $-
a054c801 584X<$-> X<$FORMAT_LINES_LEFT>
a0d0e21e
LW
585
586The number of lines left on the page of the currently selected output
19799a22
GS
587channel.
588Used with formats.
589(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 590
fe307981
GS
591=item @LAST_MATCH_START
592
6cef1e77 593=item @-
a054c801 594X<@-> X<@LAST_MATCH_START>
6cef1e77 595
19799a22 596$-[0] is the offset of the start of the last successful match.
6cef1e77 597C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 598I<n>-th subpattern, or undef if the subpattern did not match.
6cef1e77
IZ
599
600Thus after a match against $_, $& coincides with C<substr $_, $-[0],
5060ef7b
RGS
601$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
602$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
603C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
14218588
GS
604matched subgroup in the last successful match. Contrast with
605C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 606with C<@+>.
6cef1e77 607
4ba05bdc
GS
608This array holds the offsets of the beginnings of the last
609successful submatches in the currently active dynamic scope.
610C<$-[0]> is the offset into the string of the beginning of the
611entire match. The I<n>th element of this array holds the offset
0926d669
JP
612of the I<n>th submatch, so C<$-[1]> is the offset where $1
613begins, C<$-[2]> the offset where $2 begins, and so on.
4ba05bdc
GS
614
615After a match against some variable $var:
616
617=over 5
618
4375e838 619=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 620
4375e838 621=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 622
4375e838 623=item C<$'> is the same as C<substr($var, $+[0])>
4ba05bdc
GS
624
625=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
626
627=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
628
80dc6883 629=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
4ba05bdc
GS
630
631=back
632
44a2ac75
YO
633=item %-
634X<%->
635
3195cf34
RGS
636Similar to C<%+>, this variable allows access to the named capture buffers
637in the last successful match in the currently active dynamic scope. To
638each capture buffer name found in the regular expression, it associates a
639reference to an array containing the list of values captured by all
640buffers with that name (should there be several of them), in the order
641where they appear.
642
643Here's an example:
44a2ac75 644
3195cf34
RGS
645 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
646 foreach my $bufname (sort keys %-) {
647 my $ary = $-{$bufname};
44a2ac75 648 foreach my $idx (0..$#$ary) {
3195cf34 649 print "\$-{$bufname}[$idx] : ",
44a2ac75
YO
650 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
651 "\n";
652 }
653 }
654 }
655
656would print out:
657
658 $-{A}[0] : '1'
659 $-{A}[1] : '3'
660 $-{B}[0] : '2'
661 $-{B}[1] : '4'
662
3195cf34
RGS
663The keys of the C<%-> hash correspond to all buffer names found in
664the regular expression.
665
666The behaviour of C<%-> is implemented via the
80305961 667L<Tie::Hash::NamedCapture> module.
44a2ac75 668
3195cf34 669B<Note:> C<%-> and C<%+> are tied views into a common internal hash
44a2ac75
YO
670associated with the last successful regular expression. Therefore mixing
671iterative access to them via C<each> may have unpredictable results.
3195cf34 672Likewise, if the last successful match changes, then the results may be
44a2ac75
YO
673surprising.
674
fcc7d916 675=item HANDLE->format_name(EXPR)
a0d0e21e
LW
676
677=item $FORMAT_NAME
678
679=item $~
a054c801 680X<$~> X<$FORMAT_NAME>
a0d0e21e
LW
681
682The name of the current report format for the currently selected output
14218588 683channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 684C<$^>.)
a0d0e21e 685
fcc7d916 686=item HANDLE->format_top_name(EXPR)
a0d0e21e
LW
687
688=item $FORMAT_TOP_NAME
689
690=item $^
a054c801 691X<$^> X<$FORMAT_TOP_NAME>
a0d0e21e
LW
692
693The name of the current top-of-page format for the currently selected
14218588 694output channel. Default is the name of the filehandle with _TOP
a0d0e21e
LW
695appended. (Mnemonic: points to top of page.)
696
46550894 697=item IO::Handle->format_line_break_characters EXPR
a0d0e21e
LW
698
699=item $FORMAT_LINE_BREAK_CHARACTERS
700
701=item $:
a054c801 702X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e
LW
703
704The current set of characters after which a string may be broken to
54310121 705fill continuation fields (starting with ^) in a format. Default is
a0d0e21e
LW
706S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
707poetry is a part of a line.)
708
46550894 709=item IO::Handle->format_formfeed EXPR
a0d0e21e
LW
710
711=item $FORMAT_FORMFEED
712
713=item $^L
a054c801 714X<$^L> X<$FORMAT_FORMFEED>
a0d0e21e 715
14218588 716What formats output as a form feed. Default is \f.
a0d0e21e
LW
717
718=item $ACCUMULATOR
719
720=item $^A
a054c801 721X<$^A> X<$ACCUMULATOR>
a0d0e21e
LW
722
723The current value of the write() accumulator for format() lines. A format
19799a22 724contains formline() calls that put their result into C<$^A>. After
a0d0e21e 725calling its format, write() prints out the contents of C<$^A> and empties.
14218588 726So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
727formline() yourself and then look at it. See L<perlform> and
728L<perlfunc/formline()>.
729
730=item $CHILD_ERROR
731
732=item $?
a054c801 733X<$?> X<$CHILD_ERROR>
a0d0e21e 734
54310121 735The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
736successful call to wait() or waitpid(), or from the system()
737operator. This is just the 16-bit status word returned by the
e5218da5 738traditional Unix wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 739exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
740C<$? & 127> gives which signal, if any, the process died from, and
741C<$? & 128> reports whether there was a core dump. (Mnemonic:
742similar to B<sh> and B<ksh>.)
a0d0e21e 743
7b8d334a 744Additionally, if the C<h_errno> variable is supported in C, its value
14218588 745is returned via $? if any C<gethost*()> function fails.
7b8d334a 746
19799a22 747If you have installed a signal handler for C<SIGCHLD>, the
aa689395 748value of C<$?> will usually be wrong outside that handler.
749
a8f8344d 750Inside an C<END> subroutine C<$?> contains the value that is going to be
751given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
752change the exit status of your program. For example:
753
754 END {
755 $? = 1 if $? == 255; # die would make it 255
756 }
a8f8344d 757
aa689395 758Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 759actual VMS exit status, instead of the default emulation of POSIX
9bc98430 760status; see L<perlvms/$?> for details.
f86702cc 761
55602bd2
IZ
762Also see L<Error Indicators>.
763
e5218da5 764=item ${^CHILD_ERROR_NATIVE}
a054c801 765X<$^CHILD_ERROR_NATIVE>
e5218da5
GA
766
767The native status returned by the last pipe close, backtick (C<``>)
768command, successful call to wait() or waitpid(), or from the system()
769operator. On POSIX-like systems this value can be decoded with the
770WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
771and WIFCONTINUED functions provided by the L<POSIX> module.
772
773Under VMS this reflects the actual VMS exit status; i.e. it is the same
774as $? when the pragma C<use vmsish 'status'> is in effect.
775
0a378802 776=item ${^ENCODING}
a054c801 777X<$^ENCODING>
0a378802 778
740bd165
PN
779The I<object reference> to the Encode object that is used to convert
780the source code to Unicode. Thanks to this variable your perl script
781does not have to be written in UTF-8. Default is I<undef>. The direct
749e08de 782manipulation of this variable is highly discouraged.
0a378802 783
a0d0e21e
LW
784=item $OS_ERROR
785
786=item $ERRNO
787
788=item $!
a054c801 789X<$!> X<$ERRNO> X<$OS_ERROR>
a0d0e21e 790
19799a22 791If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
792variable, or in other words, if a system or library call fails, it
793sets this variable. This means that the value of C<$!> is meaningful
794only I<immediately> after a B<failure>:
795
b0169937 796 if (open my $fh, "<", $filename) {
6ab308ee
JH
797 # Here $! is meaningless.
798 ...
799 } else {
800 # ONLY here is $! meaningful.
801 ...
802 # Already here $! might be meaningless.
803 }
804 # Since here we might have either success or failure,
805 # here $! is meaningless.
806
807In the above I<meaningless> stands for anything: zero, non-zero,
808C<undef>. A successful system or library call does B<not> set
809the variable to zero.
810
271df126 811If used as a string, yields the corresponding system error string.
19799a22
GS
812You can assign a number to C<$!> to set I<errno> if, for instance,
813you want C<"$!"> to return the string for error I<n>, or you want
814to set the exit value for the die() operator. (Mnemonic: What just
815went bang?)
a0d0e21e 816
55602bd2
IZ
817Also see L<Error Indicators>.
818
daaddde1
MS
819=item %OS_ERROR
820
821=item %ERRNO
822
4c5cef9b 823=item %!
a054c801 824X<%!>
4c5cef9b
MJD
825
826Each element of C<%!> has a true value only if C<$!> is set to that
827value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
828value of C<$!> is C<ENOENT>; that is, if the most recent error was
829"No such file or directory" (or its moral equivalent: not all operating
830systems give that exact error, and certainly not all languages).
831To check if a particular key is meaningful on your system, use
832C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
833See L<Errno> for more information, and also see above for the
834validity of C<$!>.
4c5cef9b 835
5c055ba3 836=item $EXTENDED_OS_ERROR
837
838=item $^E
a054c801 839X<$^E> X<$EXTENDED_OS_ERROR>
5c055ba3 840
22fae026
TM
841Error information specific to the current operating system. At
842the moment, this differs from C<$!> under only VMS, OS/2, and Win32
843(and for MacPerl). On all other platforms, C<$^E> is always just
844the same as C<$!>.
845
846Under VMS, C<$^E> provides the VMS status value from the last
847system error. This is more specific information about the last
848system error than that provided by C<$!>. This is particularly
d516a115 849important when C<$!> is set to B<EVMSERR>.
22fae026 850
1c1c7f20
GS
851Under OS/2, C<$^E> is set to the error code of the last call to
852OS/2 API either via CRT, or directly from perl.
22fae026
TM
853
854Under Win32, C<$^E> always returns the last error information
855reported by the Win32 call C<GetLastError()> which describes
856the last error from within the Win32 API. Most Win32-specific
19799a22 857code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
858set C<errno> and so most portable Perl code will report errors
859via C<$!>.
860
861Caveats mentioned in the description of C<$!> generally apply to
862C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 863
55602bd2
IZ
864Also see L<Error Indicators>.
865
a0d0e21e
LW
866=item $EVAL_ERROR
867
868=item $@
a054c801 869X<$@> X<$EVAL_ERROR>
a0d0e21e 870
4a280ebe
JG
871The Perl syntax error message from the last eval() operator.
872If $@ is the null string, the last eval() parsed and executed
873correctly (although the operations you invoked may have failed in the
874normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 875
19799a22 876Warning messages are not collected in this variable. You can,
a8f8344d 877however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 878as described below.
748a9306 879
55602bd2
IZ
880Also see L<Error Indicators>.
881
a0d0e21e
LW
882=item $PROCESS_ID
883
884=item $PID
885
886=item $$
a054c801 887X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 888
19799a22
GS
889The process number of the Perl running this script. You should
890consider this variable read-only, although it will be altered
891across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 892
4d76a344
RGS
893Note for Linux users: on Linux, the C functions C<getpid()> and
894C<getppid()> return different values from different threads. In order to
895be portable, this behavior is not reflected by C<$$>, whose value remains
896consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 897you may use the CPAN module C<Linux::Pid>.
4d76a344 898
a0d0e21e
LW
899=item $REAL_USER_ID
900
901=item $UID
902
903=item $<
a054c801 904X<< $< >> X<$UID> X<$REAL_USER_ID>
a0d0e21e 905
19799a22 906The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685 907if you're running setuid.) You can change both the real uid and
a537debe
SP
908the effective uid at the same time by using POSIX::setuid(). Since
909changes to $< require a system call, check $! after a change attempt to
910detect any possible errors.
a0d0e21e
LW
911
912=item $EFFECTIVE_USER_ID
913
914=item $EUID
915
916=item $>
a054c801 917X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
a0d0e21e
LW
918
919The effective uid of this process. Example:
920
921 $< = $>; # set real to effective uid
922 ($<,$>) = ($>,$<); # swap real and effective uid
923
a043a685 924You can change both the effective uid and the real uid at the same
a537debe
SP
925time by using POSIX::setuid(). Changes to $> require a check to $!
926to detect any possible errors after an attempted change.
a043a685 927
19799a22 928(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 929C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 930supporting setreuid().
a0d0e21e
LW
931
932=item $REAL_GROUP_ID
933
934=item $GID
935
936=item $(
a054c801 937X<$(> X<$GID> X<$REAL_GROUP_ID>
a0d0e21e
LW
938
939The real gid of this process. If you are on a machine that supports
940membership in multiple groups simultaneously, gives a space separated
941list of groups you are in. The first number is the one returned by
942getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb 943the same as the first number.
944
19799a22
GS
945However, a value assigned to C<$(> must be a single number used to
946set the real gid. So the value given by C<$(> should I<not> be assigned
2bc0d022
DL
947back to C<$(> without being forced numeric, such as by adding zero. Note
948that this is different to the effective gid (C<$)>) which does take a
949list.
8cc95fdb 950
a043a685 951You can change both the real gid and the effective gid at the same
a537debe
SP
952time by using POSIX::setgid(). Changes to $( require a check to $!
953to detect any possible errors after an attempted change.
a043a685 954
19799a22
GS
955(Mnemonic: parentheses are used to I<group> things. The real gid is the
956group you I<left>, if you're running setgid.)
a0d0e21e
LW
957
958=item $EFFECTIVE_GROUP_ID
959
960=item $EGID
961
962=item $)
a054c801 963X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
a0d0e21e
LW
964
965The effective gid of this process. If you are on a machine that
966supports membership in multiple groups simultaneously, gives a space
967separated list of groups you are in. The first number is the one
968returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb 969which may be the same as the first number.
970
19799a22 971Similarly, a value assigned to C<$)> must also be a space-separated
14218588 972list of numbers. The first number sets the effective gid, and
8cc95fdb 973the rest (if any) are passed to setgroups(). To get the effect of an
974empty list for setgroups(), just repeat the new effective gid; that is,
975to force an effective gid of 5 and an effectively empty setgroups()
976list, say C< $) = "5 5" >.
977
a043a685
GW
978You can change both the effective gid and the real gid at the same
979time by using POSIX::setgid() (use only a single numeric argument).
a537debe
SP
980Changes to $) require a check to $! to detect any possible errors
981after an attempted change.
a043a685 982
19799a22
GS
983(Mnemonic: parentheses are used to I<group> things. The effective gid
984is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 985
c47ff5f1 986C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
987machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
988and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
989
990=item $PROGRAM_NAME
991
992=item $0
a054c801 993X<$0> X<$PROGRAM_NAME>
a0d0e21e 994
80bca1b4
JH
995Contains the name of the program being executed.
996
997On some (read: not all) operating systems assigning to C<$0> modifies
998the argument area that the C<ps> program sees. On some platforms you
999may have to use special C<ps> options or a different C<ps> to see the
1000changes. Modifying the $0 is more useful as a way of indicating the
1001current program state than it is for hiding the program you're
1002running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277 1003
cf525c36 1004Note that there are platform specific limitations on the maximum
f9cbb277
JH
1005length of C<$0>. In the most extreme case it may be limited to the
1006space occupied by the original C<$0>.
a0d0e21e 1007
80bca1b4
JH
1008In some platforms there may be arbitrary amount of padding, for
1009example space characters, after the modified name as shown by C<ps>.
dda345b7 1010In some platforms this padding may extend all the way to the original
c80e2480
JH
1011length of the argument area, no matter what you do (this is the case
1012for example with Linux 2.2).
80bca1b4 1013
4bc88a62 1014Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3
JH
1015from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
1016result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
1017and the " (perl)" suffix are shown depends on your exact BSD variant
1018and version). This is an operating system feature, Perl cannot help it.
4bc88a62 1019
e2975953
JH
1020In multithreaded scripts Perl coordinates the threads so that any
1021thread may modify its copy of the C<$0> and the change becomes visible
cf525c36 1022to ps(1) (assuming the operating system plays along). Note that
80bca1b4
JH
1023the view of C<$0> the other threads have will not change since they
1024have their own copies of it.
e2975953 1025
52c447a8
RGS
1026If the program has been given to perl via the switches C<-e> or C<-E>,
1027C<$0> will contain the string C<"-e">.
1028
a0d0e21e 1029=item $[
a054c801 1030X<$[>
a0d0e21e
LW
1031
1032The index of the first element in an array, and of the first character
19799a22
GS
1033in a substring. Default is 0, but you could theoretically set it
1034to 1 to make Perl behave more like B<awk> (or Fortran) when
1035subscripting and when evaluating the index() and substr() functions.
1036(Mnemonic: [ begins subscripts.)
a0d0e21e 1037
19799a22
GS
1038As of release 5 of Perl, assignment to C<$[> is treated as a compiler
1039directive, and cannot influence the behavior of any other file.
55b67815
RGS
1040(That's why you can only assign compile-time constants to it.) Its
1041use is deprecated, and will trigger a warning (if the deprecation
1042L<warnings> category is enabled. You did C<use warnings>, right?)
a0d0e21e 1043
f83ed198 1044Note that, unlike other compile-time directives (such as L<strict>),
af7a0647
RGS
1045assignment to C<$[> can be seen from outer lexical scopes in the same file.
1046However, you can use local() on it to strictly bind its value to a
f83ed198
RGS
1047lexical block.
1048
a0d0e21e 1049=item $]
a054c801 1050X<$]>
a0d0e21e 1051
54310121 1052The version + patchlevel / 1000 of the Perl interpreter. This variable
1053can be used to determine whether the Perl interpreter executing a
1054script is in the right range of versions. (Mnemonic: Is this version
1055of perl in the right bracket?) Example:
a0d0e21e
LW
1056
1057 warn "No checksumming!\n" if $] < 3.019;
1058
54310121 1059See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 1060for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 1061
0c8d858b
MS
1062The floating point representation can sometimes lead to inaccurate
1063numeric comparisons. See C<$^V> for a more modern representation of
1064the Perl version that allows accurate string comparisons.
16070b82 1065
305aace0
NIS
1066=item $COMPILING
1067
1068=item $^C
a054c801 1069X<$^C> X<$COMPILING>
305aace0 1070
19799a22
GS
1071The current value of the flag associated with the B<-c> switch.
1072Mainly of use with B<-MO=...> to allow code to alter its behavior
1073when being compiled, such as for example to AUTOLOAD at compile
59f521f4 1074time rather than normal, deferred loading. Setting
19799a22 1075C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 1076
a0d0e21e
LW
1077=item $DEBUGGING
1078
1079=item $^D
a054c801 1080X<$^D> X<$DEBUGGING>
a0d0e21e
LW
1081
1082The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c
DM
1083switch.) May be read or set. Like its command-line equivalent, you can use
1084numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e 1085
a3621e74
YO
1086=item ${^RE_DEBUG_FLAGS}
1087
1088The current value of the regex debugging flags. Set to 0 for no debug output
1089even when the re 'debug' module is loaded. See L<re> for details.
1090
0111c4fd 1091=item ${^RE_TRIE_MAXBUF}
a3621e74
YO
1092
1093Controls how certain regex optimisations are applied and how much memory they
1094utilize. This value by default is 65536 which corresponds to a 512kB temporary
1095cache. Set this to a higher value to trade memory for speed when matching
1096large alternations. Set it to a lower value if you want the optimisations to
1097be as conservative of memory as possible but still occur, and set it to a
1098negative value to prevent the optimisation and conserve the most memory.
1099Under normal situations this variable should be of no interest to you.
1100
a0d0e21e
LW
1101=item $SYSTEM_FD_MAX
1102
1103=item $^F
a054c801 1104X<$^F> X<$SYSTEM_FD_MAX>
a0d0e21e
LW
1105
1106The maximum system file descriptor, ordinarily 2. System file
1107descriptors are passed to exec()ed processes, while higher file
1108descriptors are not. Also, during an open(), system file descriptors are
1109preserved even if the open() fails. (Ordinary file descriptors are
19799a22 1110closed before the open() is attempted.) The close-on-exec
a0d0e21e 1111status of a file descriptor will be decided according to the value of
8d2a6795
GS
1112C<$^F> when the corresponding file, pipe, or socket was opened, not the
1113time of the exec().
a0d0e21e 1114
6e2995f4 1115=item $^H
1116
0462a1ab
GS
1117WARNING: This variable is strictly for internal use only. Its availability,
1118behavior, and contents are subject to change without notice.
1119
1120This variable contains compile-time hints for the Perl interpreter. At the
1121end of compilation of a BLOCK the value of this variable is restored to the
1122value when the interpreter started to compile the BLOCK.
1123
1124When perl begins to parse any block construct that provides a lexical scope
1125(e.g., eval body, required file, subroutine body, loop body, or conditional
1126block), the existing value of $^H is saved, but its value is left unchanged.
1127When the compilation of the block is completed, it regains the saved value.
1128Between the points where its value is saved and restored, code that
1129executes within BEGIN blocks is free to change the value of $^H.
1130
1131This behavior provides the semantic of lexical scoping, and is used in,
1132for instance, the C<use strict> pragma.
1133
1134The contents should be an integer; different bits of it are used for
1135different pragmatic flags. Here's an example:
1136
1137 sub add_100 { $^H |= 0x100 }
1138
1139 sub foo {
1140 BEGIN { add_100() }
1141 bar->baz($boon);
1142 }
1143
1144Consider what happens during execution of the BEGIN block. At this point
1145the BEGIN block has already been compiled, but the body of foo() is still
1146being compiled. The new value of $^H will therefore be visible only while
1147the body of foo() is being compiled.
1148
1149Substitution of the above BEGIN block with:
1150
1151 BEGIN { require strict; strict->import('vars') }
1152
1153demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1154version of the same lexical pragma:
1155
1156 BEGIN { require strict; strict->import('vars') if $condition }
1157
1158=item %^H
1159
0462a1ab 1160The %^H hash provides the same scoping semantic as $^H. This makes it
46e5f5f4 1161useful for implementation of lexically scoped pragmas. See L<perlpragma>.
6e2995f4 1162
a0d0e21e
LW
1163=item $INPLACE_EDIT
1164
1165=item $^I
a054c801 1166X<$^I> X<$INPLACE_EDIT>
a0d0e21e
LW
1167
1168The current value of the inplace-edit extension. Use C<undef> to disable
1169inplace editing. (Mnemonic: value of B<-i> switch.)
1170
fb73857a 1171=item $^M
a054c801 1172X<$^M>
fb73857a 1173
19799a22
GS
1174By default, running out of memory is an untrappable, fatal error.
1175However, if suitably built, Perl can use the contents of C<$^M>
1176as an emergency memory pool after die()ing. Suppose that your Perl
0acca065 1177were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
19799a22 1178Then
fb73857a 1179
19799a22 1180 $^M = 'a' x (1 << 16);
fb73857a 1181
51ee6500 1182would allocate a 64K buffer for use in an emergency. See the
19799a22 1183F<INSTALL> file in the Perl distribution for information on how to
0acca065
RGS
1184add custom C compilation flags when compiling perl. To discourage casual
1185use of this advanced feature, there is no L<English|English> long name for
1186this variable.
fb73857a 1187
5c055ba3 1188=item $OSNAME
6e2995f4 1189
5c055ba3 1190=item $^O
a054c801 1191X<$^O> X<$OSNAME>
5c055ba3 1192
1193The name of the operating system under which this copy of Perl was
1194built, as determined during the configuration process. The value
19799a22
GS
1195is identical to C<$Config{'osname'}>. See also L<Config> and the
1196B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1197
443f6d01 1198In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1199C<MSWin32>, it doesn't tell the difference between
120095/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1201Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1202between the variants.
916d64a3 1203
e2e27056
JH
1204=item ${^OPEN}
1205
1206An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1207by a C<\0> byte, the first part describes the input layers, the second
1208part describes the output layers.
e2e27056 1209
a0d0e21e
LW
1210=item $PERLDB
1211
1212=item $^P
a054c801 1213X<$^P> X<$PERLDB>
a0d0e21e 1214
19799a22
GS
1215The internal variable for debugging support. The meanings of the
1216various bits are subject to change, but currently indicate:
84902520
TB
1217
1218=over 6
1219
1220=item 0x01
1221
1222Debug subroutine enter/exit.
1223
1224=item 0x02
1225
4c85b59c
TB
1226Line-by-line debugging. Causes DB::DB() subroutine to be called for each
1227statement executed. Also causes saving source code lines (like 0x400).
84902520
TB
1228
1229=item 0x04
1230
1231Switch off optimizations.
1232
1233=item 0x08
1234
1235Preserve more data for future interactive inspections.
1236
1237=item 0x10
1238
1239Keep info about source lines on which a subroutine is defined.
1240
1241=item 0x20
1242
1243Start with single-step on.
1244
83ee9e09
GS
1245=item 0x40
1246
1247Use subroutine address instead of name when reporting.
1248
1249=item 0x80
1250
1251Report C<goto &subroutine> as well.
1252
1253=item 0x100
1254
1255Provide informative "file" names for evals based on the place they were compiled.
1256
1257=item 0x200
1258
1259Provide informative names to anonymous subroutines based on the place they
1260were compiled.
1261
7619c85e
RG
1262=item 0x400
1263
4c85b59c 1264Save source code lines into C<@{"_<$filename"}>.
7619c85e 1265
84902520
TB
1266=back
1267
19799a22
GS
1268Some bits may be relevant at compile-time only, some at
1269run-time only. This is a new mechanism and the details may change.
4c85b59c 1270See also L<perldebguts>.
a0d0e21e 1271
66558a10
GS
1272=item $LAST_REGEXP_CODE_RESULT
1273
b9ac3b5b 1274=item $^R
a054c801 1275X<$^R> X<$LAST_REGEXP_CODE_RESULT>
b9ac3b5b 1276
19799a22
GS
1277The result of evaluation of the last successful C<(?{ code })>
1278regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1279
66558a10
GS
1280=item $EXCEPTIONS_BEING_CAUGHT
1281
fb73857a 1282=item $^S
a054c801 1283X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
fb73857a 1284
fa05a9fd
IT
1285Current state of the interpreter.
1286
1287 $^S State
1288 --------- -------------------
1289 undef Parsing module/eval
1290 true (1) Executing an eval
1291 false (0) Otherwise
1292
1293The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1294
a0d0e21e
LW
1295=item $BASETIME
1296
1297=item $^T
a054c801 1298X<$^T> X<$BASETIME>
a0d0e21e 1299
19799a22 1300The time at which the program began running, in seconds since the
5f05dabc 1301epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1302and B<-C> filetests are based on this value.
a0d0e21e 1303
7c36658b
MS
1304=item ${^TAINT}
1305
9aa05f58
RGS
1306Reflects if taint mode is on or off. 1 for on (the program was run with
1307B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1308B<-t> or B<-TU>). This variable is read-only.
7c36658b 1309
a05d7ebb
JH
1310=item ${^UNICODE}
1311
ab9e1bb7
JH
1312Reflects certain Unicode settings of Perl. See L<perlrun>
1313documentation for the C<-C> switch for more information about
1314the possible values. This variable is set during Perl startup
1315and is thereafter read-only.
fde18df1 1316
e07ea26a
NC
1317=item ${^UTF8CACHE}
1318
1319This variable controls the state of the internal UTF-8 offset caching code.
16d9fe92
NC
13201 for on (the default), 0 for off, -1 to debug the caching code by checking
1321all its results against linear scans, and panicking on any discrepancy.
e07ea26a 1322
ea8eae40
RGS
1323=item ${^UTF8LOCALE}
1324
1325This variable indicates whether an UTF-8 locale was detected by perl at
1326startup. This information is used by perl when it's in
1327adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1328switch); see L<perlrun> for more info on this.
1329
44dcb63b 1330=item $PERL_VERSION
b459063d 1331
16070b82 1332=item $^V
a054c801 1333X<$^V> X<$PERL_VERSION>
16070b82
GS
1334
1335The revision, version, and subversion of the Perl interpreter, represented
a32521b7 1336as a C<version> object.
16070b82 1337
7d2b1222 1338This variable first appeared in perl 5.6.0; earlier versions of perl will
a32521b7 1339see an undefined value. Before perl 5.10.0 $^V was represented as a v-string.
7d2b1222 1340
a32521b7 1341$^V can be used to determine whether the Perl interpreter executing a
16070b82 1342script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1343Control.) Example:
16070b82 1344
7d2b1222 1345 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
16070b82 1346
aa2f2a36
AMS
1347To convert C<$^V> into its string representation use sprintf()'s
1348C<"%vd"> conversion:
1349
1350 printf "version is v%vd\n", $^V; # Perl's version
1351
44dcb63b 1352See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1353for a convenient way to fail if the running Perl interpreter is too old.
1354
1355See also C<$]> for an older representation of the Perl version.
1356
a0d0e21e
LW
1357=item $WARNING
1358
1359=item $^W
a054c801 1360X<$^W> X<$WARNING>
a0d0e21e 1361
19799a22
GS
1362The current value of the warning switch, initially true if B<-w>
1363was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1364related to the B<-w> switch.) See also L<warnings>.
1365
6a818117 1366=item ${^WARNING_BITS}
4438c4b7
JH
1367
1368The current set of warning checks enabled by the C<use warnings> pragma.
1369See the documentation of C<warnings> for more details.
a0d0e21e 1370
2a8c8378
JD
1371=item ${^WIN32_SLOPPY_STAT}
1372
1373If this variable is set to a true value, then stat() on Windows will
1374not try to open the file. This means that the link count cannot be
1375determined and file attributes may be out of date if additional
1376hardlinks to the file exist. On the other hand, not opening the file
1377is considerably faster, especially for files on network drives.
1378
1379This variable could be set in the F<sitecustomize.pl> file to
1380configure the local Perl installation to use "sloppy" stat() by
1381default. See L<perlrun> for more information about site
1382customization.
1383
a0d0e21e
LW
1384=item $EXECUTABLE_NAME
1385
1386=item $^X
a054c801 1387X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 1388
e71940de 1389The name used to execute the current copy of Perl, from C's
21c1191d 1390C<argv[0]> or (where supported) F</proc/self/exe>.
38e4f4ae 1391
e71940de
PG
1392Depending on the host operating system, the value of $^X may be
1393a relative or absolute pathname of the perl program file, or may
1394be the string used to invoke perl but not the pathname of the
1395perl program file. Also, most operating systems permit invoking
1396programs that are not in the PATH environment variable, so there
a10d74f3
PG
1397is no guarantee that the value of $^X is in PATH. For VMS, the
1398value may or may not include a version number.
38e4f4ae 1399
e71940de
PG
1400You usually can use the value of $^X to re-invoke an independent
1401copy of the same perl that is currently running, e.g.,
1402
1403 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1404
1405But recall that not all operating systems support forking or
1406capturing of the output of commands, so this complex statement
1407may not be portable.
38e4f4ae 1408
e71940de
PG
1409It is not safe to use the value of $^X as a path name of a file,
1410as some operating systems that have a mandatory suffix on
1411executable files do not require use of the suffix when invoking
1412a command. To convert the value of $^X to a path name, use the
1413following statements:
1414
304dea91 1415 # Build up a set of file names (not command names).
e71940de 1416 use Config;
68fb0eb7
PG
1417 $this_perl = $^X;
1418 if ($^O ne 'VMS')
1419 {$this_perl .= $Config{_exe}
1420 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1421
1422Because many operating systems permit anyone with read access to
1423the Perl program file to make a copy of it, patch the copy, and
1424then execute the copy, the security-conscious Perl programmer
1425should take care to invoke the installed copy of perl, not the
1426copy referenced by $^X. The following statements accomplish
1427this goal, and produce a pathname that can be invoked as a
1428command or referenced as a file.
38e4f4ae
SB
1429
1430 use Config;
68fb0eb7
PG
1431 $secure_perl_path = $Config{perlpath};
1432 if ($^O ne 'VMS')
1433 {$secure_perl_path .= $Config{_exe}
1434 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1435
2d84a16a 1436=item ARGV
a054c801 1437X<ARGV>
2d84a16a
DM
1438
1439The special filehandle that iterates over command-line filenames in
1440C<@ARGV>. Usually written as the null filehandle in the angle operator
1441C<< <> >>. Note that currently C<ARGV> only has its magical effect
1442within the C<< <> >> operator; elsewhere it is just a plain filehandle
1443corresponding to the last file opened by C<< <> >>. In particular,
1444passing C<\*ARGV> as a parameter to a function that expects a filehandle
1445may not cause your function to automatically read the contents of all the
1446files in C<@ARGV>.
1447
a0d0e21e 1448=item $ARGV
a054c801 1449X<$ARGV>
a0d0e21e 1450
c47ff5f1 1451contains the name of the current file when reading from <>.
a0d0e21e
LW
1452
1453=item @ARGV
a054c801 1454X<@ARGV>
a0d0e21e 1455
19799a22 1456The array @ARGV contains the command-line arguments intended for
14218588 1457the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1458one, because C<$ARGV[0]> is the first argument, I<not> the program's
1459command name itself. See C<$0> for the command name.
a0d0e21e 1460
5ccee41e 1461=item ARGVOUT
a054c801 1462X<ARGVOUT>
5ccee41e
JA
1463
1464The special filehandle that points to the currently open output file
1465when doing edit-in-place processing with B<-i>. Useful when you have
1466to do a lot of inserting and don't want to keep modifying $_. See
1467L<perlrun> for the B<-i> switch.
1468
9b0e6e7a 1469=item @F
a054c801 1470X<@F>
9b0e6e7a
JP
1471
1472The array @F contains the fields of each line read in when autosplit
1473mode is turned on. See L<perlrun> for the B<-a> switch. This array
1474is package-specific, and must be declared or given a full package name
1475if not in package main when running under C<strict 'vars'>.
1476
a0d0e21e 1477=item @INC
a054c801 1478X<@INC>
a0d0e21e 1479
19799a22
GS
1480The array @INC contains the list of places that the C<do EXPR>,
1481C<require>, or C<use> constructs look for their library files. It
1482initially consists of the arguments to any B<-I> command-line
1483switches, followed by the default Perl library, probably
1484F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1485directory. ("." will not be appended if taint checks are enabled, either by
1486C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1487the C<use lib> pragma to get the machine-dependent library properly
1488loaded also:
a0d0e21e 1489
cb1a09d0
AD
1490 use lib '/mypath/libdir/';
1491 use SomeMod;
303f2f76 1492
d54b56d5
RGS
1493You can also insert hooks into the file inclusion system by putting Perl
1494code directly into @INC. Those hooks may be subroutine references, array
1495references or blessed objects. See L<perlfunc/require> for details.
1496
314d39ce
MG
1497=item @ARG
1498
fb73857a 1499=item @_
a054c801 1500X<@_> X<@ARG>
fb73857a 1501
1502Within a subroutine the array @_ contains the parameters passed to that
19799a22 1503subroutine. See L<perlsub>.
fb73857a 1504
a0d0e21e 1505=item %INC
a054c801 1506X<%INC>
a0d0e21e 1507
19799a22
GS
1508The hash %INC contains entries for each filename included via the
1509C<do>, C<require>, or C<use> operators. The key is the filename
1510you specified (with module names converted to pathnames), and the
14218588 1511value is the location of the file found. The C<require>
87275199 1512operator uses this hash to determine whether a particular file has
19799a22 1513already been included.
a0d0e21e 1514
89ccab8c
RGS
1515If the file was loaded via a hook (e.g. a subroutine reference, see
1516L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1517by default inserted into %INC in place of a filename. Note, however,
1518that the hook may have set the %INC entry by itself to provide some more
1519specific info.
44f0be63 1520
b687b08b
TC
1521=item %ENV
1522
1523=item $ENV{expr}
a054c801 1524X<%ENV>
a0d0e21e
LW
1525
1526The hash %ENV contains your current environment. Setting a
19799a22
GS
1527value in C<ENV> changes the environment for any child processes
1528you subsequently fork() off.
a0d0e21e 1529
b687b08b
TC
1530=item %SIG
1531
1532=item $SIG{expr}
a054c801 1533X<%SIG>
a0d0e21e 1534
efbd929d 1535The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e
LW
1536
1537 sub handler { # 1st argument is signal name
fb73857a 1538 my($sig) = @_;
a0d0e21e
LW
1539 print "Caught a SIG$sig--shutting down\n";
1540 close(LOG);
1541 exit(0);
1542 }
1543
fb73857a 1544 $SIG{'INT'} = \&handler;
1545 $SIG{'QUIT'} = \&handler;
a0d0e21e 1546 ...
19799a22 1547 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1548 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1549
f648820c
GS
1550Using a value of C<'IGNORE'> usually has the effect of ignoring the
1551signal, except for the C<CHLD> signal. See L<perlipc> for more about
1552this special case.
1553
19799a22 1554Here are some other examples:
a0d0e21e 1555
fb73857a 1556 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1557 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1558 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1559 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1560
19799a22
GS
1561Be sure not to use a bareword as the name of a signal handler,
1562lest you inadvertently call it.
748a9306 1563
44a8e56a 1564If your system has the sigaction() function then signal handlers are
9ce5b4ad 1565installed using it. This means you get reliable signal handling.
44a8e56a 1566
9ce5b4ad
SG
1567The default delivery policy of signals changed in Perl 5.8.0 from
1568immediate (also known as "unsafe") to deferred, also known as
1569"safe signals". See L<perlipc> for more information.
45c0772f 1570
748a9306 1571Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1572routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306 1573about to be printed. The warning message is passed as the first
efbd929d
AT
1574argument. The presence of a C<__WARN__> hook causes the ordinary printing
1575of warnings to C<STDERR> to be suppressed. You can use this to save warnings
748a9306
LW
1576in a variable, or turn warnings into fatal errors, like this:
1577
1578 local $SIG{__WARN__} = sub { die $_[0] };
1579 eval $proggie;
1580
efbd929d
AT
1581As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
1582disable warnings using the empty subroutine:
1583
1584 local $SIG{__WARN__} = sub {};
1585
a8f8344d 1586The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306 1587is about to be thrown. The error message is passed as the first
efbd929d 1588argument. When a C<__DIE__> hook routine returns, the exception
748a9306 1589processing continues as it would have in the absence of the hook,
efbd929d 1590unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>.
774d564b 1591The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a 1592can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1593
19799a22
GS
1594Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1595even inside an eval(). Do not use this to rewrite a pending exception
efbd929d 1596in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>.
19799a22
GS
1597This strange action at a distance may be fixed in a future release
1598so that C<$SIG{__DIE__}> is only called if your program is about
1599to exit, as was the original intent. Any other use is deprecated.
1600
1601C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1602they may be called to report (probable) errors found by the parser.
1603In such a case the parser may be in inconsistent state, so any
1604attempt to evaluate Perl code from such a handler will probably
1605result in a segfault. This means that warnings or errors that
1606result from parsing Perl should be used with extreme caution, like
1607this:
fb73857a 1608
1609 require Carp if defined $^S;
1610 Carp::confess("Something wrong") if defined &Carp::confess;
1611 die "Something wrong, but could not load Carp to give backtrace...
1612 To see backtrace try starting Perl with -MCarp switch";
1613
1614Here the first line will load Carp I<unless> it is the parser who
1615called the handler. The second line will print backtrace and die if
1616Carp was available. The third line will be executed only if Carp was
1617not available.
1618
19799a22 1619See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1620L<warnings> for additional information.
68dc0745 1621
a0d0e21e 1622=back
55602bd2
IZ
1623
1624=head2 Error Indicators
a054c801 1625X<error> X<exception>
55602bd2 1626
19799a22
GS
1627The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1628about different types of error conditions that may appear during
1629execution of a Perl program. The variables are shown ordered by
1630the "distance" between the subsystem which reported the error and
1631the Perl process. They correspond to errors detected by the Perl
1632interpreter, C library, operating system, or an external program,
1633respectively.
55602bd2
IZ
1634
1635To illustrate the differences between these variables, consider the
19799a22 1636following Perl expression, which uses a single-quoted string:
55602bd2 1637
19799a22 1638 eval q{
22d0716c
SB
1639 open my $pipe, "/cdrom/install |" or die $!;
1640 my @res = <$pipe>;
1641 close $pipe or die "bad pipe: $?, $!";
19799a22 1642 };
55602bd2
IZ
1643
1644After execution of this statement all 4 variables may have been set.
1645
19799a22
GS
1646C<$@> is set if the string to be C<eval>-ed did not compile (this
1647may happen if C<open> or C<close> were imported with bad prototypes),
1648or if Perl code executed during evaluation die()d . In these cases
1649the value of $@ is the compile error, or the argument to C<die>
4cb1c523 1650(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
19799a22
GS
1651though.)
1652
c47ff5f1 1653When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1654and C<close> are translated to calls in the C run-time library and
1655thence to the operating system kernel. C<$!> is set to the C library's
1656C<errno> if one of these calls fails.
1657
1658Under a few operating systems, C<$^E> may contain a more verbose
1659error indicator, such as in this case, "CDROM tray not closed."
14218588 1660Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1661the same as C<$!>.
1662
1663Finally, C<$?> may be set to non-0 value if the external program
1664F</cdrom/install> fails. The upper eight bits reflect specific
1665error conditions encountered by the program (the program's exit()
1666value). The lower eight bits reflect mode of failure, like signal
1667death and core dump information See wait(2) for details. In
1668contrast to C<$!> and C<$^E>, which are set only if error condition
1669is detected, the variable C<$?> is set on each C<wait> or pipe
1670C<close>, overwriting the old value. This is more like C<$@>, which
1671on every eval() is always set on failure and cleared on success.
2b92dfce 1672
19799a22
GS
1673For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1674and C<$?>.
2b92dfce
GS
1675
1676=head2 Technical Note on the Syntax of Variable Names
1677
19799a22
GS
1678Variable names in Perl can have several formats. Usually, they
1679must begin with a letter or underscore, in which case they can be
1680arbitrarily long (up to an internal limit of 251 characters) and
1681may contain letters, digits, underscores, or the special sequence
1682C<::> or C<'>. In this case, the part before the last C<::> or
1683C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1684
1685Perl variable names may also be a sequence of digits or a single
1686punctuation or control character. These names are all reserved for
19799a22
GS
1687special uses by Perl; for example, the all-digits names are used
1688to hold data captured by backreferences after a regular expression
1689match. Perl has a special syntax for the single-control-character
1690names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1691character. For example, the notation C<$^W> (dollar-sign caret
1692C<W>) is the scalar variable whose name is the single character
1693control-C<W>. This is better than typing a literal control-C<W>
1694into your program.
2b92dfce 1695
87275199 1696Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1697strings that begin with control characters (or better yet, a caret).
1698These variables must be written in the form C<${^Foo}>; the braces
1699are not optional. C<${^Foo}> denotes the scalar variable whose
1700name is a control-C<F> followed by two C<o>'s. These variables are
1701reserved for future special uses by Perl, except for the ones that
1702begin with C<^_> (control-underscore or caret-underscore). No
1703control-character name that begins with C<^_> will acquire a special
1704meaning in any future version of Perl; such names may therefore be
1705used safely in programs. C<$^_> itself, however, I<is> reserved.
1706
1fcb18de
RGS
1707Perl identifiers that begin with digits, control characters, or
1708punctuation characters are exempt from the effects of the C<package>
1709declaration and are always forced to be in package C<main>; they are
1710also exempt from C<strict 'vars'> errors. A few other names are also
1711exempt in these ways:
2b92dfce
GS
1712
1713 ENV STDIN
1714 INC STDOUT
1715 ARGV STDERR
5b88253b 1716 ARGVOUT _
2b92dfce
GS
1717 SIG
1718
1719In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1720to be in package C<main>, regardless of any C<package> declarations
747fafda 1721presently in scope.
2b92dfce 1722
19799a22
GS
1723=head1 BUGS
1724
1725Due to an unfortunate accident of Perl's implementation, C<use
1726English> imposes a considerable performance penalty on all regular
1727expression matches in a program, regardless of whether they occur
1728in the scope of C<use English>. For that reason, saying C<use
1729English> in libraries is strongly discouraged. See the
1730Devel::SawAmpersand module documentation from CPAN
1577cd80 1731( http://www.cpan.org/modules/by-module/Devel/ )
a054c801
GS
1732for more information. Writing C<use English '-no_match_vars';>
1733avoids the performance penalty.
2b92dfce 1734
19799a22
GS
1735Having to even think about the C<$^S> variable in your exception
1736handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1737invites grievous and difficult to track down errors. Avoid it
1738and use an C<END{}> or CORE::GLOBAL::die override instead.