1PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
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6 perltrap - Perl traps for the unwary
7
9 The biggest trap of all is forgetting to "use warnings" or use the -w
10 switch; see perllexwarn and perlrun. The second biggest trap is not
11 making your entire program runnable under "use strict". The third
12 biggest trap is not reading the list of changes in this version of
13 Perl; see perldelta.
14 Awk Traps
16 Accustomed awk users should take special note of the following:
17 · A Perl program executes only once, not once for each input line.
19 You can do an implicit loop with "-n" or "-p".
20 · The English module, loaded via
22 use English;
24 allows you to refer to special variables (like $/) with names (like
26 $RS), as though they were in awk; see perlvar for details.
27 · Semicolons are required after all simple statements in Perl (except
29 at the end of a block). Newline is not a statement delimiter.
30 · Curly brackets are required on "if"s and "while"s.
32 · Variables begin with "$", "@" or "%" in Perl.
34 · Arrays index from 0. Likewise string positions in substr() and
36 index().
37 · You have to decide whether your array has numeric or string
39 indices.
40 · Hash values do not spring into existence upon mere reference.
42 · You have to decide whether you want to use string or numeric
44 comparisons.
45 · Reading an input line does not split it for you. You get to split
47 it to an array yourself. And the split() operator has different
48 arguments than awk's.
49 · The current input line is normally in $_, not $0. It generally
51 does not have the newline stripped. ($0 is the name of the program
52 executed.) See perlvar.
53 · $<digit> does not refer to fields--it refers to substrings matched
55 by the last match pattern.
56 · The print() statement does not add field and record separators
58 unless you set $, and "$\". You can set $OFS and $ORS if you're
59 using the English module.
60 · You must open your files before you print to them.
62 · The range operator is "..", not comma. The comma operator works as
64 in C.
65 · The match operator is "=~", not "~". ("~" is the one's complement
67 operator, as in C.)
68 · The exponentiation operator is "**", not "^". "^" is the XOR
70 operator, as in C. (You know, one could get the feeling that awk
71 is basically incompatible with C.)
72 · The concatenation operator is ".", not the null string. (Using the
74 null string would render "/pat/ /pat/" unparsable, because the
75 third slash would be interpreted as a division operator--the
76 tokenizer is in fact slightly context sensitive for operators like
77 "/", "?", and ">". And in fact, "." itself can be the beginning of
78 a number.)
79 · The "next", "exit", and "continue" keywords work differently.
81 · The following variables work differently:
83 Awk Perl
85 ARGC scalar @ARGV (compare with $#ARGV)
86 ARGV[0] $0
87 FILENAME $ARGV
88 FNR $. - something
89 FS (whatever you like)
90 NF $#Fld, or some such
91 NR $.
92 OFMT $#
93 OFS $,
94 ORS $\
95 RLENGTH length($&)
96 RS $/
97 RSTART length($`)
98 SUBSEP $;
99 · You cannot set $RS to a pattern, only a string.
101 · When in doubt, run the awk construct through a2p and see what it
103 gives you.
104 C/C++ Traps
106 Cerebral C and C++ programmers should take note of the following:
107 · Curly brackets are required on "if"'s and "while"'s.
109 · You must use "elsif" rather than "else if".
111 · The "break" and "continue" keywords from C become in Perl "last"
113 and "next", respectively. Unlike in C, these do not work within a
114 "do { } while" construct. See "Loop Control" in perlsyn.
115 · The switch statement is called "given/when" and only available in
117 perl 5.10 or newer. See "Switch statements" in perlsyn.
118 · Variables begin with "$", "@" or "%" in Perl.
120 · Comments begin with "#", not "/*" or "//". Perl may interpret
122 C/C++ comments as division operators, unterminated regular
123 expressions or the defined-or operator.
124 · You can't take the address of anything, although a similar operator
126 in Perl is the backslash, which creates a reference.
127 · "ARGV" must be capitalized. $ARGV[0] is C's "argv[1]", and
129 "argv[0]" ends up in $0.
130 · System calls such as link(), unlink(), rename(), etc. return
132 nonzero for success, not 0. (system(), however, returns zero for
133 success.)
134 · Signal handlers deal with signal names, not numbers. Use "kill -l"
136 to find their names on your system.
137 Sed Traps
139 Seasoned sed programmers should take note of the following:
140 · A Perl program executes only once, not once for each input line.
142 You can do an implicit loop with "-n" or "-p".
143 · Backreferences in substitutions use "$" rather than "\".
145 · The pattern matching metacharacters "(", ")", and "|" do not have
147 backslashes in front.
148 · The range operator is "...", rather than comma.
150 Shell Traps
152 Sharp shell programmers should take note of the following:
153 · The backtick operator does variable interpolation without regard to
155 the presence of single quotes in the command.
156 · The backtick operator does no translation of the return value,
158 unlike csh.
159 · Shells (especially csh) do several levels of substitution on each
161 command line. Perl does substitution in only certain constructs
162 such as double quotes, backticks, angle brackets, and search
163 patterns.
164 · Shells interpret scripts a little bit at a time. Perl compiles the
166 entire program before executing it (except for "BEGIN" blocks,
167 which execute at compile time).
168 · The arguments are available via @ARGV, not $1, $2, etc.
170 · The environment is not automatically made available as separate
172 scalar variables.
173 · The shell's "test" uses "=", "!=", "<" etc for string comparisons
175 and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the
176 reverse of Perl, which uses "eq", "ne", "lt" for string
177 comparisons, and "==", "!=" "<" etc for numeric comparisons.
178 Perl Traps
180 Practicing Perl Programmers should take note of the following:
181 · Remember that many operations behave differently in a list context
183 than they do in a scalar one. See perldata for details.
184 · Avoid barewords if you can, especially all lowercase ones. You
186 can't tell by just looking at it whether a bareword is a function
187 or a string. By using quotes on strings and parentheses on
188 function calls, you won't ever get them confused.
189 · You cannot discern from mere inspection which builtins are unary
191 operators (like chop() and chdir()) and which are list operators
192 (like print() and unlink()). (Unless prototyped, user-defined
193 subroutines can only be list operators, never unary ones.) See
194 perlop and perlsub.
195 · People have a hard time remembering that some functions default to
197 $_, or @ARGV, or whatever, but that others which you might expect
198 to do not.
199 · The <FH> construct is not the name of the filehandle, it is a
201 readline operation on that handle. The data read is assigned to $_
202 only if the file read is the sole condition in a while loop:
203 while (<FH>) { }
205 while (defined($_ = <FH>)) { }..
206 <FH>; # data discarded!
207 · Remember not to use "=" when you need "=~"; these two constructs
209 are quite different:
210 $x = /foo/;
212 $x =~ /foo/;
213 · The "do {}" construct isn't a real loop that you can use loop
215 control on.
216 · Use "my()" for local variables whenever you can get away with it
218 (but see perlform for where you can't). Using "local()" actually
219 gives a local value to a global variable, which leaves you open to
220 unforeseen side-effects of dynamic scoping.
221 · If you localize an exported variable in a module, its exported
223 value will not change. The local name becomes an alias to a new
224 value but the external name is still an alias for the original.
225 Perl4 to Perl5 Traps
227 Practicing Perl4 Programmers should take note of the following
228 Perl4-to-Perl5 specific traps.
229 They're crudely ordered according to the following list:
231 Discontinuance, Deprecation, and BugFix traps
233 Anything that's been fixed as a perl4 bug, removed as a perl4
234 feature or deprecated as a perl4 feature with the intent to
235 encourage usage of some other perl5 feature.
236 Parsing Traps
238 Traps that appear to stem from the new parser.
239 Numerical Traps
241 Traps having to do with numerical or mathematical operators.
242 General data type traps
244 Traps involving perl standard data types.
245 Context Traps - scalar, list contexts
247 Traps related to context within lists, scalar
248 statements/declarations.
249 Precedence Traps
251 Traps related to the precedence of parsing, evaluation, and
252 execution of code.
253 General Regular Expression Traps using s///, etc.
255 Traps related to the use of pattern matching.
256 Subroutine, Signal, Sorting Traps
258 Traps related to the use of signals and signal handlers, general
259 subroutines, and sorting, along with sorting subroutines.
260 OS Traps
262 OS-specific traps.
263 DBM Traps
265 Traps specific to the use of "dbmopen()", and specific dbm
266 implementations.
267 Unclassified Traps
269 Everything else.
270 If you find an example of a conversion trap that is not listed here,
272 please submit it to <perlbug@perl.org> for inclusion. Also note that
273 at least some of these can be caught with the "use warnings" pragma or
274 the -w switch.
275 Discontinuance, Deprecation, and BugFix traps
277 Anything that has been discontinued, deprecated, or fixed as a bug from
278 perl4.
279 · Symbols starting with "_" no longer forced into main
281 Symbols starting with "_" are no longer forced into package main,
283 except for $_ itself (and @_, etc.).
284 package test;
286 $_legacy = 1;
287 package main;
289 print "\$_legacy is ",$_legacy,"\n";
290 # perl4 prints: $_legacy is 1
292 # perl5 prints: $_legacy is
293 · Double-colon valid package separator in variable name
295 Double-colon is now a valid package separator in a variable name.
297 Thus these behave differently in perl4 vs. perl5, because the
298 packages don't exist.
299 $a=1;$b=2;$c=3;$var=4;
301 print "$a::$b::$c ";
302 print "$var::abc::xyz\n";
303 # perl4 prints: 1::2::3 4::abc::xyz
305 # perl5 prints: 3
306 Given that "::" is now the preferred package delimiter, it is
308 debatable whether this should be classed as a bug or not. (The
309 older package delimiter, ' ,is used here)
310 $x = 10;
312 print "x=${'x}\n";
313 # perl4 prints: x=10
315 # perl5 prints: Can't find string terminator "'" anywhere before EOF
316 You can avoid this problem, and remain compatible with perl4, if
318 you always explicitly include the package name:
319 $x = 10;
321 print "x=${main'x}\n";
322 Also see precedence traps, for parsing $:.
324 · 2nd and 3rd args to "splice()" are now in scalar context
326 The second and third arguments of "splice()" are now evaluated in
328 scalar context (as the Camel says) rather than list context.
329 sub sub1{return(0,2) } # return a 2-element list
331 sub sub2{ return(1,2,3)} # return a 3-element list
332 @a1 = ("a","b","c","d","e");
333 @a2 = splice(@a1,&sub1,&sub2);
334 print join(' ',@a2),"\n";
335 # perl4 prints: a b
337 # perl5 prints: c d e
338 · Can't do "goto" into a block that is optimized away
340 You can't do a "goto" into a block that is optimized away. Darn.
342 goto marker1;
344 for(1){
346 marker1:
347 print "Here I is!\n";
348 }
349 # perl4 prints: Here I is!
351 # perl5 errors: Can't "goto" into the middle of a foreach loop
352 · Can't use whitespace as variable name or quote delimiter
354 It is no longer syntactically legal to use whitespace as the name
356 of a variable, or as a delimiter for any kind of quote construct.
357 Double darn.
358 $a = ("foo bar");
360 $b = q baz;
361 print "a is $a, b is $b\n";
362 # perl4 prints: a is foo bar, b is baz
364 # perl5 errors: Bareword found where operator expected
365 · "while/if BLOCK BLOCK" gone
367 The archaic while/if BLOCK BLOCK syntax is no longer supported.
369 if { 1 } {
371 print "True!";
372 }
373 else {
374 print "False!";
375 }
376 # perl4 prints: True!
378 # perl5 errors: syntax error at test.pl line 1, near "if {"
379 · "**" binds tighter than unary minus
381 The "**" operator now binds more tightly than unary minus. It was
383 documented to work this way before, but didn't.
384 print -4**2,"\n";
386 # perl4 prints: 16
388 # perl5 prints: -16
389 · "foreach" changed when iterating over a list
391 The meaning of "foreach{}" has changed slightly when it is
393 iterating over a list which is not an array. This used to assign
394 the list to a temporary array, but no longer does so (for
395 efficiency). This means that you'll now be iterating over the
396 actual values, not over copies of the values. Modifications to the
397 loop variable can change the original values.
398 @list = ('ab','abc','bcd','def');
400 foreach $var (grep(/ab/,@list)){
401 $var = 1;
402 }
403 print (join(':',@list));
404 # perl4 prints: ab:abc:bcd:def
406 # perl5 prints: 1:1:bcd:def
407 To retain Perl4 semantics you need to assign your list explicitly
409 to a temporary array and then iterate over that. For example, you
410 might need to change
411 foreach $var (grep(/ab/,@list)){
413 to
415 foreach $var (@tmp = grep(/ab/,@list)){
417 Otherwise changing $var will clobber the values of @list. (This
419 most often happens when you use $_ for the loop variable, and call
420 subroutines in the loop that don't properly localize $_.)
421 · "split" with no args behavior changed
423 "split" with no arguments now behaves like "split ' '" (which
425 doesn't return an initial null field if $_ starts with whitespace),
426 it used to behave like "split /\s+/" (which does).
427 $_ = ' hi mom';
429 print join(':', split);
430 # perl4 prints: :hi:mom
432 # perl5 prints: hi:mom
433 · -e behavior fixed
435 Perl 4 would ignore any text which was attached to an -e switch,
437 always taking the code snippet from the following arg.
438 Additionally, it would silently accept an -e switch without a
439 following arg. Both of these behaviors have been fixed.
440 perl -e'print "attached to -e"' 'print "separate arg"'
442 # perl4 prints: separate arg
444 # perl5 prints: attached to -e
445 perl -e
447 # perl4 prints:
449 # perl5 dies: No code specified for -e.
450 · "push" returns number of elements in resulting list
452 In Perl 4 the return value of "push" was undocumented, but it was
454 actually the last value being pushed onto the target list. In Perl
455 5 the return value of "push" is documented, but has changed, it is
456 the number of elements in the resulting list.
457 @x = ('existing');
459 print push(@x, 'first new', 'second new');
460 # perl4 prints: second new
462 # perl5 prints: 3
463 · Some error messages differ
465 Some error messages will be different.
467 · "split()" honors subroutine args
469 In Perl 4, if in list context the delimiters to the first argument
471 of "split()" were "??", the result would be placed in @_ as well as
472 being returned. Perl 5 has more respect for your subroutine
473 arguments.
474 · Bugs removed
476 Some bugs may have been inadvertently removed. :-)
478 Parsing Traps
480 Perl4-to-Perl5 traps from having to do with parsing.
481 · Space between . and = triggers syntax error
483 Note the space between . and =
485 $string . = "more string";
487 print $string;
488 # perl4 prints: more string
490 # perl5 prints: syntax error at - line 1, near ". ="
491 · Better parsing in perl 5
493 Better parsing in perl 5
495 sub foo {}
497 &foo
498 print("hello, world\n");
499 # perl4 prints: hello, world
501 # perl5 prints: syntax error
502 · Function parsing
504 "if it looks like a function, it is a function" rule.
506 print
508 ($foo == 1) ? "is one\n" : "is zero\n";
509 # perl4 prints: is zero
511 # perl5 warns: "Useless use of a constant in void context" if using -w
512 · String interpolation of $#array differs
514 String interpolation of the $#array construct differs when braces
516 are to used around the name.
517 @a = (1..3);
519 print "${#a}";
520 # perl4 prints: 2
522 # perl5 fails with syntax error
523 @ = (1..3);
525 print "$#{a}";
526 # perl4 prints: {a}
528 # perl5 prints: 2
529 · Perl guesses on "map", "grep" followed by "{" if it starts BLOCK or
531 hash ref
532 When perl sees "map {" (or "grep {"), it has to guess whether the
534 "{" starts a BLOCK or a hash reference. If it guesses wrong, it
535 will report a syntax error near the "}" and the missing (or
536 unexpected) comma.
537 Use unary "+" before "{" on a hash reference, and unary "+" applied
539 to the first thing in a BLOCK (after "{"), for perl to guess right
540 all the time. (See "map" in perlfunc.)
541 Numerical Traps
543 Perl4-to-Perl5 traps having to do with numerical operators, operands,
544 or output from same.
545 · Formatted output and significant digits
547 Formatted output and significant digits. In general, Perl 5 tries
549 to be more precise. For example, on a Solaris Sparc:
550 print 7.373504 - 0, "\n";
552 printf "%20.18f\n", 7.373504 - 0;
553 # Perl4 prints:
555 7.3750399999999996141
556 7.375039999999999614
557 # Perl5 prints:
559 7.373504
560 7.375039999999999614
561 Notice how the first result looks better in Perl 5.
563 Your results may vary, since your floating point formatting
565 routines and even floating point format may be slightly different.
566 · Auto-increment operator over signed int limit deleted
568 This specific item has been deleted. It demonstrated how the
570 auto-increment operator would not catch when a number went over
571 the signed int limit. Fixed in version 5.003_04. But always be
572 wary when using large integers. If in doubt:
573 use Math::BigInt;
575 · Assignment of return values from numeric equality tests doesn't
577 work
578 Assignment of return values from numeric equality tests does not
580 work in perl5 when the test evaluates to false (0). Logical tests
581 now return a null, instead of 0
582 $p = ($test == 1);
584 print $p,"\n";
585 # perl4 prints: 0
587 # perl5 prints:
588 Also see "//, etc."" in "General Regular Expression Traps using s
590 for another example of this new feature...
591 · Bitwise string ops
593 When bitwise operators which can operate upon either numbers or
595 strings ("& | ^ ~") are given only strings as arguments, perl4
596 would treat the operands as bitstrings so long as the program
597 contained a call to the "vec()" function. perl5 treats the string
598 operands as bitstrings. (See "Bitwise String Operators" in perlop
599 for more details.)
600 $fred = "10";
602 $barney = "12";
603 $betty = $fred & $barney;
604 print "$betty\n";
605 # Uncomment the next line to change perl4's behavior
606 # ($dummy) = vec("dummy", 0, 0);
607 # Perl4 prints:
609 8
610 # Perl5 prints:
612 10
613 # If vec() is used anywhere in the program, both print:
615 10
616 General data type traps
618 Perl4-to-Perl5 traps involving most data-types, and their usage within
619 certain expressions and/or context.
620 · Negative array subscripts now count from the end of array
622 Negative array subscripts now count from the end of the array.
624 @a = (1, 2, 3, 4, 5);
626 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
627 # perl4 prints: The third element of the array is 4 also expressed as
629 # perl5 prints: The third element of the array is 4 also expressed as 4
630 · Setting $#array lower now discards array elements
632 Setting $#array lower now discards array elements, and makes them
634 impossible to recover.
635 @a = (a,b,c,d,e);
637 print "Before: ",join('',@a);
638 $#a =1;
639 print ", After: ",join('',@a);
640 $#a =3;
641 print ", Recovered: ",join('',@a),"\n";
642 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
644 # perl5 prints: Before: abcde, After: ab, Recovered: ab
645 · Hashes get defined before use
647 Hashes get defined before use
649 local($s,@a,%h);
651 die "scalar \$s defined" if defined($s);
652 die "array \@a defined" if defined(@a);
653 die "hash \%h defined" if defined(%h);
654 # perl4 prints:
656 # perl5 dies: hash %h defined
657 Perl will now generate a warning when it sees defined(@a) and
659 defined(%h).
660 · Glob assignment from localized variable to variable
662 glob assignment from variable to variable will fail if the
664 assigned variable is localized subsequent to the assignment
665 @a = ("This is Perl 4");
667 *b = *a;
668 local(@a);
669 print @b,"\n";
670 # perl4 prints: This is Perl 4
672 # perl5 prints:
673 · Assigning "undef" to glob
675 Assigning "undef" to a glob has no effect in Perl 5. In Perl 4
677 it undefines the associated scalar (but may have other side
678 effects including SEGVs). Perl 5 will also warn if "undef" is
679 assigned to a typeglob. (Note that assigning "undef" to a typeglob
680 is different than calling the "undef" function on a typeglob
681 ("undef *foo"), which has quite a few effects.
682 $foo = "bar";
684 *foo = undef;
685 print $foo;
686 # perl4 prints:
688 # perl4 warns: "Use of uninitialized variable" if using -w
689 # perl5 prints: bar
690 # perl5 warns: "Undefined value assigned to typeglob" if using -w
691 · Changes in unary negation (of strings)
693 Changes in unary negation (of strings) This change effects both
695 the return value and what it does to auto(magic)increment.
696 $x = "aaa";
698 print ++$x," : ";
699 print -$x," : ";
700 print ++$x,"\n";
701 # perl4 prints: aab : -0 : 1
703 # perl5 prints: aab : -aab : aac
704 · Modifying of constants prohibited
706 perl 4 lets you modify constants:
708 $foo = "x";
710 &mod($foo);
711 for ($x = 0; $x < 3; $x++) {
712 &mod("a");
713 }
714 sub mod {
715 print "before: $_[0]";
716 $_[0] = "m";
717 print " after: $_[0]\n";
718 }
719 # perl4:
721 # before: x after: m
722 # before: a after: m
723 # before: m after: m
724 # before: m after: m
725 # Perl5:
727 # before: x after: m
728 # Modification of a read-only value attempted at foo.pl line 12.
729 # before: a
730 · "defined $var" behavior changed
732 The behavior is slightly different for:
734 print "$x", defined $x
736 # perl 4: 1
738 # perl 5: <no output, $x is not called into existence>
739 · Variable Suicide
741 Variable suicide behavior is more consistent under Perl 5. Perl5
743 exhibits the same behavior for hashes and scalars, that perl4
744 exhibits for only scalars.
745 $aGlobal{ "aKey" } = "global value";
747 print "MAIN:", $aGlobal{"aKey"}, "\n";
748 $GlobalLevel = 0;
749 &test( *aGlobal );
750 sub test {
752 local( *theArgument ) = @_;
753 local( %aNewLocal ); # perl 4 != 5.001l,m
754 $aNewLocal{"aKey"} = "this should never appear";
755 print "SUB: ", $theArgument{"aKey"}, "\n";
756 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
757 $GlobalLevel++;
758 if( $GlobalLevel<4 ) {
759 &test( *aNewLocal );
760 }
761 }
762 # Perl4:
764 # MAIN:global value
765 # SUB: global value
766 # SUB: level 0
767 # SUB: level 1
768 # SUB: level 2
769 # Perl5:
771 # MAIN:global value
772 # SUB: global value
773 # SUB: this should never appear
774 # SUB: this should never appear
775 # SUB: this should never appear
776 Context Traps - scalar, list contexts
778 · Elements of argument lists for formats evaluated in list context
779 The elements of argument lists for formats are now evaluated in
781 list context. This means you can interpolate list values now.
782 @fmt = ("foo","bar","baz");
784 format STDOUT=
785 @<<<<< @||||| @>>>>>
786 @fmt;
787 .
788 write;
789 # perl4 errors: Please use commas to separate fields in file
791 # perl5 prints: foo bar baz
792 · "caller()" returns false value in scalar context if no caller
794 present
795 The "caller()" function now returns a false value in a scalar
797 context if there is no caller. This lets library files determine
798 if they're being required.
799 caller() ? (print "You rang?\n") : (print "Got a 0\n");
801 # perl4 errors: There is no caller
803 # perl5 prints: Got a 0
804 · Comma operator in scalar context gives scalar context to args
806 The comma operator in a scalar context is now guaranteed to give a
808 scalar context to its arguments.
809 @y= ('a','b','c');
811 $x = (1, 2, @y);
812 print "x = $x\n";
813 # Perl4 prints: x = c # Thinks list context interpolates list
815 # Perl5 prints: x = 3 # Knows scalar uses length of list
816 · "sprintf()" prototyped as "($;@)"
818 "sprintf()" is prototyped as ($;@), so its first argument is given
820 scalar context. Thus, if passed an array, it will probably not do
821 what you want, unlike Perl 4:
822 @z = ('%s%s', 'foo', 'bar');
824 $x = sprintf(@z);
825 print $x;
826 # perl4 prints: foobar
828 # perl5 prints: 3
829 "printf()" works the same as it did in Perl 4, though:
831 @z = ('%s%s', 'foo', 'bar');
833 printf STDOUT (@z);
834 # perl4 prints: foobar
836 # perl5 prints: foobar
837 Precedence Traps
839 Perl4-to-Perl5 traps involving precedence order.
840 Perl 4 has almost the same precedence rules as Perl 5 for the operators
842 that they both have. Perl 4 however, seems to have had some
843 inconsistencies that made the behavior differ from what was documented.
844 · LHS vs. RHS of any assignment operator
846 LHS vs. RHS of any assignment operator. LHS is evaluated first in
848 perl4, second in perl5; this can affect the relationship between
849 side-effects in sub-expressions.
850 @arr = ( 'left', 'right' );
852 $a{shift @arr} = shift @arr;
853 print join( ' ', keys %a );
854 # perl4 prints: left
856 # perl5 prints: right
857 · Semantic errors introduced due to precedence
859 These are now semantic errors because of precedence:
861 @list = (1,2,3,4,5);
863 %map = ("a",1,"b",2,"c",3,"d",4);
864 $n = shift @list + 2; # first item in list plus 2
865 print "n is $n, ";
866 $m = keys %map + 2; # number of items in hash plus 2
867 print "m is $m\n";
868 # perl4 prints: n is 3, m is 6
870 # perl5 errors and fails to compile
871 · Precedence of assignment operators same as the precedence of
873 assignment
874 The precedence of assignment operators is now the same as the
876 precedence of assignment. Perl 4 mistakenly gave them the
877 precedence of the associated operator. So you now must
878 parenthesize them in expressions like
879 /foo/ ? ($a += 2) : ($a -= 2);
881 Otherwise
883 /foo/ ? $a += 2 : $a -= 2
885 would be erroneously parsed as
887 (/foo/ ? $a += 2 : $a) -= 2;
889 On the other hand,
891 $a += /foo/ ? 1 : 2;
893 now works as a C programmer would expect.
895 · "open" requires parentheses around filehandle
897 open FOO || die;
899 is now incorrect. You need parentheses around the filehandle.
901 Otherwise, perl5 leaves the statement as its default precedence:
902 open(FOO || die);
904 # perl4 opens or dies
906 # perl5 opens FOO, dying only if 'FOO' is false, i.e. never
907 · $: precedence over $:: gone
909 perl4 gives the special variable, $: precedence, where perl5
911 treats $:: as main "package"
912 $a = "x"; print "$::a";
914 # perl 4 prints: -:a
916 # perl 5 prints: x
917 · Precedence of file test operators documented
919 perl4 had buggy precedence for the file test operators vis-a-vis
921 the assignment operators. Thus, although the precedence table for
922 perl4 leads one to believe "-e $foo .= "q"" should parse as "((-e
923 $foo) .= "q")", it actually parses as "(-e ($foo .= "q"))". In
924 perl5, the precedence is as documented.
925 -e $foo .= "q"
927 # perl4 prints: no output
929 # perl5 prints: Can't modify -e in concatenation
930 · "keys", "each", "values" are regular named unary operators
932 In perl4, keys(), each() and values() were special high-precedence
934 operators that operated on a single hash, but in perl5, they are
935 regular named unary operators. As documented, named unary
936 operators have lower precedence than the arithmetic and
937 concatenation operators "+ - .", but the perl4 variants of these
938 operators actually bind tighter than "+ - .". Thus, for:
939 %foo = 1..10;
941 print keys %foo - 1
942 # perl4 prints: 4
944 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
945 The perl4 behavior was probably more useful, if less consistent.
947 General Regular Expression Traps using s///, etc.
949 All types of RE traps.
950 · "s'$lhs'$rhs'" interpolates on either side
952 "s'$lhs'$rhs'" now does no interpolation on either side. It used
954 to interpolate $lhs but not $rhs. (And still does not match a
955 literal '$' in string)
956 $a=1;$b=2;
958 $string = '1 2 $a $b';
959 $string =~ s'$a'$b';
960 print $string,"\n";
961 # perl4 prints: $b 2 $a $b
963 # perl5 prints: 1 2 $a $b
964 · "m//g" attaches its state to the searched string
966 "m//g" now attaches its state to the searched string rather than
968 the regular expression. (Once the scope of a block is left for
969 the sub, the state of the searched string is lost)
970 $_ = "ababab";
972 while(m/ab/g){
973 &doit("blah");
974 }
975 sub doit{local($_) = shift; print "Got $_ "}
976 # perl4 prints: Got blah Got blah Got blah Got blah
978 # perl5 prints: infinite loop blah...
979 · "m//o" used within an anonymous sub
981 Currently, if you use the "m//o" qualifier on a regular expression
983 within an anonymous sub, all closures generated from that
984 anonymous sub will use the regular expression as it was compiled
985 when it was used the very first time in any such closure. For
986 instance, if you say
987 sub build_match {
989 my($left,$right) = @_;
990 return sub { $_[0] =~ /$left stuff $right/o; };
991 }
992 $good = build_match('foo','bar');
993 $bad = build_match('baz','blarch');
994 print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
995 print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
996 print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
997 For most builds of Perl5, this will print: ok not ok not ok
999 build_match() will always return a sub which matches the contents
1001 of $left and $right as they were the first time that build_match()
1002 was called, not as they are in the current call.
1003 · $+ isn't set to whole match
1005 If no parentheses are used in a match, Perl4 sets $+ to the whole
1007 match, just like $&. Perl5 does not.
1008 "abcdef" =~ /b.*e/;
1010 print "\$+ = $+\n";
1011 # perl4 prints: bcde
1013 # perl5 prints:
1014 · Substitution now returns null string if it fails
1016 substitution now returns the null string if it fails
1018 $string = "test";
1020 $value = ($string =~ s/foo//);
1021 print $value, "\n";
1022 # perl4 prints: 0
1024 # perl5 prints:
1025 Also see "Numerical Traps" for another example of this new
1027 feature.
1028 · "s`lhs`rhs`" is now a normal substitution
1030 "s`lhs`rhs`" (using backticks) is now a normal substitution, with
1032 no backtick expansion
1033 $string = "";
1035 $string =~ s`^`hostname`;
1036 print $string, "\n";
1037 # perl4 prints: <the local hostname>
1039 # perl5 prints: hostname
1040 · Stricter parsing of variables in regular expressions
1042 Stricter parsing of variables used in regular expressions
1044 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1046 # perl4: compiles w/o error
1048 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1049 an added component of this example, apparently from the same
1051 script, is the actual value of the s'd string after the
1052 substitution. "[$opt]" is a character class in perl4 and an array
1053 subscript in perl5
1054 $grpc = 'a';
1056 $opt = 'r';
1057 $_ = 'bar';
1058 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1059 print;
1060 # perl4 prints: foo
1062 # perl5 prints: foobar
1063 · "m?x?" matches only once
1065 Under perl5, "m?x?" matches only once, like "?x?". Under perl4, it
1067 matched repeatedly, like "/x/" or "m!x!".
1068 $test = "once";
1070 sub match { $test =~ m?once?; }
1071 &match();
1072 if( &match() ) {
1073 # m?x? matches more then once
1074 print "perl4\n";
1075 } else {
1076 # m?x? matches only once
1077 print "perl5\n";
1078 }
1079 # perl4 prints: perl4
1081 # perl5 prints: perl5
1082 · Failed matches don't reset the match variables
1084 Unlike in Ruby, failed matches in Perl do not reset the match
1086 variables ($1, $2, ..., "$`", ...).
1087 Subroutine, Signal, Sorting Traps
1089 The general group of Perl4-to-Perl5 traps having to do with Signals,
1090 Sorting, and their related subroutines, as well as general subroutine
1091 traps. Includes some OS-Specific traps.
1092 · Barewords that used to look like strings look like subroutine
1094 calls
1095 Barewords that used to look like strings to Perl will now look
1097 like subroutine calls if a subroutine by that name is defined
1098 before the compiler sees them.
1099 sub SeeYa { warn"Hasta la vista, baby!" }
1101 $SIG{'TERM'} = SeeYa;
1102 print "SIGTERM is now $SIG{'TERM'}\n";
1103 # perl4 prints: SIGTERM is now main'SeeYa
1105 # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
1106 Use -w to catch this one
1108 · Reverse is no longer allowed as the name of a sort subroutine
1110 reverse is no longer allowed as the name of a sort subroutine.
1112 sub reverse{ print "yup "; $a <=> $b }
1114 print sort reverse (2,1,3);
1115 # perl4 prints: yup yup 123
1117 # perl5 prints: 123
1118 # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()
1119 · "warn()" won't let you specify a filehandle.
1121 Although it _always_ printed to STDERR, warn() would let you
1123 specify a filehandle in perl4. With perl5 it does not.
1124 warn STDERR "Foo!";
1126 # perl4 prints: Foo!
1128 # perl5 prints: String found where operator expected
1129 OS Traps
1131 · SysV resets signal handler correctly
1132 Under HPUX, and some other SysV OSes, one had to reset any signal
1134 handler, within the signal handler function, each time a signal
1135 was handled with perl4. With perl5, the reset is now done
1136 correctly. Any code relying on the handler _not_ being reset will
1137 have to be reworked.
1138 Since version 5.002, Perl uses sigaction() under SysV.
1140 sub gotit {
1142 print "Got @_... ";
1143 }
1144 $SIG{'INT'} = 'gotit';
1145 $| = 1;
1147 $pid = fork;
1148 if ($pid) {
1149 kill('INT', $pid);
1150 sleep(1);
1151 kill('INT', $pid);
1152 } else {
1153 while (1) {sleep(10);}
1154 }
1155 # perl4 (HPUX) prints: Got INT...
1157 # perl5 (HPUX) prints: Got INT... Got INT...
1158 · SysV "seek()" appends correctly
1160 Under SysV OSes, "seek()" on a file opened to append ">>" now does
1162 the right thing w.r.t. the fopen() manpage. e.g., - When a file is
1163 opened for append, it is impossible to overwrite information
1164 already in the file.
1165 open(TEST,">>seek.test");
1167 $start = tell TEST;
1168 foreach(1 .. 9){
1169 print TEST "$_ ";
1170 }
1171 $end = tell TEST;
1172 seek(TEST,$start,0);
1173 print TEST "18 characters here";
1174 # perl4 (solaris) seek.test has: 18 characters here
1176 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1177 Interpolation Traps
1179 Perl4-to-Perl5 traps having to do with how things get interpolated
1180 within certain expressions, statements, contexts, or whatever.
1181 · "@" always interpolates an array in double-quotish strings
1183 @ now always interpolates an array in double-quotish strings.
1185 print "To: someone@somewhere.com\n";
1187 # perl4 prints: To:someone@somewhere.com
1189 # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
1190 # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
1191 · Double-quoted strings may no longer end with an unescaped $
1193 Double-quoted strings may no longer end with an unescaped $.
1195 $foo = "foo$";
1197 print "foo is $foo\n";
1198 # perl4 prints: foo is foo$
1200 # perl5 errors: Final $ should be \$ or $name
1201 Note: perl5 DOES NOT error on the terminating @ in $bar
1203 · Arbitrary expressions are evaluated inside braces within double
1205 quotes
1206 Perl now sometimes evaluates arbitrary expressions inside braces
1208 that occur within double quotes (usually when the opening brace is
1209 preceded by "$" or "@").
1210 @www = "buz";
1212 $foo = "foo";
1213 $bar = "bar";
1214 sub foo { return "bar" };
1215 print "|@{w.w.w}|${main'foo}|";
1216 # perl4 prints: |@{w.w.w}|foo|
1218 # perl5 prints: |buz|bar|
1219 Note that you can "use strict;" to ward off such trappiness under
1221 perl5.
1222 · $$x now tries to dereference $x
1224 The construct "this is $$x" used to interpolate the pid at that
1226 point, but now tries to dereference $x. $$ by itself still works
1227 fine, however.
1228 $s = "a reference";
1230 $x = *s;
1231 print "this is $$x\n";
1232 # perl4 prints: this is XXXx (XXX is the current pid)
1234 # perl5 prints: this is a reference
1235 · Creation of hashes on the fly with "eval "EXPR"" requires
1237 protection
1238 Creation of hashes on the fly with "eval "EXPR"" now requires
1240 either both "$"'s to be protected in the specification of the hash
1241 name, or both curlies to be protected. If both curlies are
1242 protected, the result will be compatible with perl4 and perl5.
1243 This is a very common practice, and should be changed to use the
1244 block form of "eval{}" if possible.
1245 $hashname = "foobar";
1247 $key = "baz";
1248 $value = 1234;
1249 eval "\$$hashname{'$key'} = q|$value|";
1250 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1251 # perl4 prints: Yup
1253 # perl5 prints: Nope
1254 Changing
1256 eval "\$$hashname{'$key'} = q|$value|";
1258 to
1260 eval "\$\$hashname{'$key'} = q|$value|";
1262 causes the following result:
1264 # perl4 prints: Nope
1266 # perl5 prints: Yup
1267 or, changing to
1269 eval "\$$hashname\{'$key'\} = q|$value|";
1271 causes the following result:
1273 # perl4 prints: Yup
1275 # perl5 prints: Yup
1276 # and is compatible for both versions
1277 · Bugs in earlier perl versions
1279 perl4 programs which unconsciously rely on the bugs in earlier
1281 perl versions.
1282 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1284 # perl4 prints: This is not perl5
1286 # perl5 prints: This is perl5
1287 · Array and hash brackets during interpolation
1289 You also have to be careful about array and hash brackets during
1291 interpolation.
1292 print "$foo["
1294 perl 4 prints: [
1296 perl 5 prints: syntax error
1297 print "$foo{"
1299 perl 4 prints: {
1301 perl 5 prints: syntax error
1302 Perl 5 is expecting to find an index or key name following the
1304 respective brackets, as well as an ending bracket of the
1305 appropriate type. In order to mimic the behavior of Perl 4, you
1306 must escape the bracket like so.
1307 print "$foo\[";
1309 print "$foo\{";
1310 · Interpolation of "\$$foo{bar}"
1312 Similarly, watch out for: "\$$foo{bar}"
1314 $foo = "baz";
1316 print "\$$foo{bar}\n";
1317 # perl4 prints: $baz{bar}
1319 # perl5 prints: $
1320 Perl 5 is looking for $foo{bar} which doesn't exist, but perl 4 is
1322 happy just to expand $foo to "baz" by itself. Watch out for this
1323 especially in "eval"'s.
1324 · "qq()" string passed to "eval" will not find string terminator
1326 "qq()" string passed to "eval"
1328 eval qq(
1330 foreach \$y (keys %\$x\) {
1331 \$count++;
1332 }
1333 );
1334 # perl4 runs this ok
1336 # perl5 prints: Can't find string terminator ")"
1337 DBM Traps
1339 General DBM traps.
1340 · Perl5 must have been linked with same dbm/ndbm as the default for
1342 "dbmopen()"
1343 Existing dbm databases created under perl4 (or any other dbm/ndbm
1345 tool) may cause the same script, run under perl5, to fail. The
1346 build of perl5 must have been linked with the same dbm/ndbm as the
1347 default for "dbmopen()" to function properly without "tie"'ing to
1348 an extension dbm implementation.
1349 dbmopen (%dbm, "file", undef);
1351 print "ok\n";
1352 # perl4 prints: ok
1354 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1355 · DBM exceeding limit on the key/value size will cause perl5 to exit
1357 immediately
1358 Existing dbm databases created under perl4 (or any other dbm/ndbm
1360 tool) may cause the same script, run under perl5, to fail. The
1361 error generated when exceeding the limit on the key/value size
1362 will cause perl5 to exit immediately.
1363 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1365 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1366 print "YUP\n";
1367 # perl4 prints:
1369 dbm store returned -1, errno 28, key "trap" at - line 3.
1370 YUP
1371 # perl5 prints:
1373 dbm store returned -1, errno 28, key "trap" at - line 3.
1374 Unclassified Traps
1376 Everything else.
1377 · "require"/"do" trap using returned value
1379 If the file doit.pl has:
1381 sub foo {
1383 $rc = do "./do.pl";
1384 return 8;
1385 }
1386 print &foo, "\n";
1387 And the do.pl file has the following single line:
1389 return 3;
1391 Running doit.pl gives the following:
1393 # perl 4 prints: 3 (aborts the subroutine early)
1395 # perl 5 prints: 8
1396 Same behavior if you replace "do" with "require".
1398 · "split" on empty string with LIMIT specified
1400 $string = '';
1402 @list = split(/foo/, $string, 2)
1403 Perl4 returns a one element list containing the empty string but
1405 Perl5 returns an empty list.
1406 As always, if any of these are ever officially declared as bugs,
1408 they'll be fixed and removed.
1409perl v5.12.4 2011-06-07 PERLTRAP(1)