3 perltrap - Perl traps for the unwary
7 The biggest trap of all is forgetting to C<use warnings> or use the B<-w>
8 switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not
9 making your entire program runnable under C<use strict>. The third biggest
10 trap is not reading the list of changes in this version of Perl; see
15 Accustomed B<awk> users should take special note of the following:
21 The English module, loaded via
25 allows you to refer to special variables (like C<$/>) with names (like
26 $RS), as though they were in B<awk>; see L<perlvar> for details.
30 Semicolons are required after all simple statements in Perl (except
31 at the end of a block). Newline is not a statement delimiter.
35 Curly brackets are required on C<if>s and C<while>s.
39 Variables begin with "$", "@" or "%" in Perl.
43 Arrays index from 0. Likewise string positions in substr() and
48 You have to decide whether your array has numeric or string indices.
52 Hash values do not spring into existence upon mere reference.
56 You have to decide whether you want to use string or numeric
61 Reading an input line does not split it for you. You get to split it
62 to an array yourself. And the split() operator has different
63 arguments than B<awk>'s.
67 The current input line is normally in $_, not $0. It generally does
68 not have the newline stripped. ($0 is the name of the program
69 executed.) See L<perlvar>.
73 $<I<digit>> does not refer to fields--it refers to substrings matched
74 by the last match pattern.
78 The print() statement does not add field and record separators unless
79 you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using
84 You must open your files before you print to them.
88 The range operator is "..", not comma. The comma operator works as in
93 The match operator is "=~", not "~". ("~" is the one's complement
98 The exponentiation operator is "**", not "^". "^" is the XOR
99 operator, as in C. (You know, one could get the feeling that B<awk> is
100 basically incompatible with C.)
104 The concatenation operator is ".", not the null string. (Using the
105 null string would render C</pat/ /pat/> unparsable, because the third slash
106 would be interpreted as a division operator--the tokenizer is in fact
107 slightly context sensitive for operators like "/", "?", and ">".
108 And in fact, "." itself can be the beginning of a number.)
112 The C<next>, C<exit>, and C<continue> keywords work differently.
117 The following variables work differently:
120 ARGC scalar @ARGV (compare with $#ARGV)
124 FS (whatever you like)
125 NF $#Fld, or some such
137 You cannot set $RS to a pattern, only a string.
141 When in doubt, run the B<awk> construct through B<a2p> and see what it
148 Cerebral C programmers should take note of the following:
154 Curly brackets are required on C<if>'s and C<while>'s.
158 You must use C<elsif> rather than C<else if>.
162 The C<break> and C<continue> keywords from C become in
163 Perl C<last> and C<next>, respectively.
164 Unlike in C, these do I<not> work within a C<do { } while> construct.
168 There's no switch statement. (But it's easy to build one on the fly.)
172 Variables begin with "$", "@" or "%" in Perl.
176 Comments begin with "#", not "/*".
180 You can't take the address of anything, although a similar operator
181 in Perl is the backslash, which creates a reference.
185 C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
190 System calls such as link(), unlink(), rename(), etc. return nonzero for
191 success, not 0. (system(), however, returns zero for success.)
195 Signal handlers deal with signal names, not numbers. Use C<kill -l>
196 to find their names on your system.
202 Seasoned B<sed> programmers should take note of the following:
208 Backreferences in substitutions use "$" rather than "\".
212 The pattern matching metacharacters "(", ")", and "|" do not have backslashes
217 The range operator is C<...>, rather than comma.
223 Sharp shell programmers should take note of the following:
229 The backtick operator does variable interpolation without regard to
230 the presence of single quotes in the command.
234 The backtick operator does no translation of the return value, unlike B<csh>.
238 Shells (especially B<csh>) do several levels of substitution on each
239 command line. Perl does substitution in only certain constructs
240 such as double quotes, backticks, angle brackets, and search patterns.
244 Shells interpret scripts a little bit at a time. Perl compiles the
245 entire program before executing it (except for C<BEGIN> blocks, which
246 execute at compile time).
250 The arguments are available via @ARGV, not $1, $2, etc.
254 The environment is not automatically made available as separate scalar
261 Practicing Perl Programmers should take note of the following:
267 Remember that many operations behave differently in a list
268 context than they do in a scalar one. See L<perldata> for details.
272 Avoid barewords if you can, especially all lowercase ones.
273 You can't tell by just looking at it whether a bareword is
274 a function or a string. By using quotes on strings and
275 parentheses on function calls, you won't ever get them confused.
279 You cannot discern from mere inspection which builtins
280 are unary operators (like chop() and chdir())
281 and which are list operators (like print() and unlink()).
282 (Unless prototyped, user-defined subroutines can B<only> be list
283 operators, never unary ones.) See L<perlop> and L<perlsub>.
287 People have a hard time remembering that some functions
288 default to $_, or @ARGV, or whatever, but that others which
289 you might expect to do not.
293 The <FH> construct is not the name of the filehandle, it is a readline
294 operation on that handle. The data read is assigned to $_ only if the
295 file read is the sole condition in a while loop:
298 while (defined($_ = <FH>)) { }..
299 <FH>; # data discarded!
303 Remember not to use C<=> when you need C<=~>;
304 these two constructs are quite different:
311 The C<do {}> construct isn't a real loop that you can use
316 Use C<my()> for local variables whenever you can get away with
317 it (but see L<perlform> for where you can't).
318 Using C<local()> actually gives a local value to a global
319 variable, which leaves you open to unforeseen side-effects
324 If you localize an exported variable in a module, its exported value will
325 not change. The local name becomes an alias to a new value but the
326 external name is still an alias for the original.
330 =head2 Perl4 to Perl5 Traps
332 Practicing Perl4 Programmers should take note of the following
333 Perl4-to-Perl5 specific traps.
335 They're crudely ordered according to the following list:
339 =item Discontinuance, Deprecation, and BugFix traps
341 Anything that's been fixed as a perl4 bug, removed as a perl4 feature
342 or deprecated as a perl4 feature with the intent to encourage usage of
343 some other perl5 feature.
347 Traps that appear to stem from the new parser.
349 =item Numerical Traps
351 Traps having to do with numerical or mathematical operators.
353 =item General data type traps
355 Traps involving perl standard data types.
357 =item Context Traps - scalar, list contexts
359 Traps related to context within lists, scalar statements/declarations.
361 =item Precedence Traps
363 Traps related to the precedence of parsing, evaluation, and execution of
366 =item General Regular Expression Traps using s///, etc.
368 Traps related to the use of pattern matching.
370 =item Subroutine, Signal, Sorting Traps
372 Traps related to the use of signals and signal handlers, general subroutines,
373 and sorting, along with sorting subroutines.
381 Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
383 =item Unclassified Traps
389 If you find an example of a conversion trap that is not listed here,
390 please submit it to <F<perlbug@perl.org>> for inclusion.
391 Also note that at least some of these can be caught with the
392 C<use warnings> pragma or the B<-w> switch.
394 =head2 Discontinuance, Deprecation, and BugFix traps
396 Anything that has been discontinued, deprecated, or fixed as
401 =item * Discontinuance
403 Symbols starting with "_" are no longer forced into package main, except
404 for C<$_> itself (and C<@_>, etc.).
410 print "\$_legacy is ",$_legacy,"\n";
412 # perl4 prints: $_legacy is 1
413 # perl5 prints: $_legacy is
417 Double-colon is now a valid package separator in a variable name. Thus these
418 behave differently in perl4 vs. perl5, because the packages don't exist.
420 $a=1;$b=2;$c=3;$var=4;
422 print "$var::abc::xyz\n";
424 # perl4 prints: 1::2::3 4::abc::xyz
427 Given that C<::> is now the preferred package delimiter, it is debatable
428 whether this should be classed as a bug or not.
429 (The older package delimiter, ' ,is used here)
435 # perl5 prints: Can't find string terminator "'" anywhere before EOF
437 You can avoid this problem, and remain compatible with perl4, if you
438 always explicitly include the package name:
441 print "x=${main'x}\n" ;
443 Also see precedence traps, for parsing C<$:>.
447 The second and third arguments of C<splice()> are now evaluated in scalar
448 context (as the Camel says) rather than list context.
450 sub sub1{return(0,2) } # return a 2-element list
451 sub sub2{ return(1,2,3)} # return a 3-element list
452 @a1 = ("a","b","c","d","e");
453 @a2 = splice(@a1,&sub1,&sub2);
454 print join(' ',@a2),"\n";
457 # perl5 prints: c d e
459 =item * Discontinuance
461 You can't do a C<goto> into a block that is optimized away. Darn.
467 print "Here I is!\n";
470 # perl4 prints: Here I is!
471 # perl5 errors: Can't "goto" into the middle of a foreach loop
473 =item * Discontinuance
475 It is no longer syntactically legal to use whitespace as the name
476 of a variable, or as a delimiter for any kind of quote construct.
481 print "a is $a, b is $b\n";
483 # perl4 prints: a is foo bar, b is baz
484 # perl5 errors: Bareword found where operator expected
486 =item * Discontinuance
488 The archaic while/if BLOCK BLOCK syntax is no longer supported.
497 # perl4 prints: True!
498 # perl5 errors: syntax error at test.pl line 1, near "if {"
502 The C<**> operator now binds more tightly than unary minus.
503 It was documented to work this way before, but didn't.
510 =item * Discontinuance
512 The meaning of C<foreach{}> has changed slightly when it is iterating over a
513 list which is not an array. This used to assign the list to a
514 temporary array, but no longer does so (for efficiency). This means
515 that you'll now be iterating over the actual values, not over copies of
516 the values. Modifications to the loop variable can change the original
519 @list = ('ab','abc','bcd','def');
520 foreach $var (grep(/ab/,@list)){
523 print (join(':',@list));
525 # perl4 prints: ab:abc:bcd:def
526 # perl5 prints: 1:1:bcd:def
528 To retain Perl4 semantics you need to assign your list
529 explicitly to a temporary array and then iterate over that. For
530 example, you might need to change
532 foreach $var (grep(/ab/,@list)){
536 foreach $var (@tmp = grep(/ab/,@list)){
538 Otherwise changing $var will clobber the values of @list. (This most often
539 happens when you use C<$_> for the loop variable, and call subroutines in
540 the loop that don't properly localize C<$_>.)
542 =item * Discontinuance
544 C<split> with no arguments now behaves like C<split ' '> (which doesn't
545 return an initial null field if $_ starts with whitespace), it used to
546 behave like C<split /\s+/> (which does).
549 print join(':', split);
551 # perl4 prints: :hi:mom
552 # perl5 prints: hi:mom
556 Perl 4 would ignore any text which was attached to an B<-e> switch,
557 always taking the code snippet from the following arg. Additionally, it
558 would silently accept an B<-e> switch without a following arg. Both of
559 these behaviors have been fixed.
561 perl -e'print "attached to -e"' 'print "separate arg"'
563 # perl4 prints: separate arg
564 # perl5 prints: attached to -e
569 # perl5 dies: No code specified for -e.
571 =item * Discontinuance
573 In Perl 4 the return value of C<push> was undocumented, but it was
574 actually the last value being pushed onto the target list. In Perl 5
575 the return value of C<push> is documented, but has changed, it is the
576 number of elements in the resulting list.
579 print push(@x, 'first new', 'second new');
581 # perl4 prints: second new
586 Some error messages will be different.
588 =item * Discontinuance
590 In Perl 4, if in list context the delimiters to the first argument of
591 C<split()> were C<??>, the result would be placed in C<@_> as well as
592 being returned. Perl 5 has more respect for your subroutine arguments.
594 =item * Discontinuance
596 Some bugs may have been inadvertently removed. :-)
602 Perl4-to-Perl5 traps from having to do with parsing.
608 Note the space between . and =
610 $string . = "more string";
613 # perl4 prints: more string
614 # perl5 prints: syntax error at - line 1, near ". ="
618 Better parsing in perl 5
622 print("hello, world\n");
624 # perl4 prints: hello, world
625 # perl5 prints: syntax error
629 "if it looks like a function, it is a function" rule.
632 ($foo == 1) ? "is one\n" : "is zero\n";
634 # perl4 prints: is zero
635 # perl5 warns: "Useless use of a constant in void context" if using -w
639 String interpolation of the C<$#array> construct differs when braces
640 are to used around the name.
646 # perl5 fails with syntax error
656 =head2 Numerical Traps
658 Perl4-to-Perl5 traps having to do with numerical operators,
659 operands, or output from same.
665 Formatted output and significant digits
667 print 7.373504 - 0, "\n";
668 printf "%20.18f\n", 7.373504 - 0;
680 This specific item has been deleted. It demonstrated how the auto-increment
681 operator would not catch when a number went over the signed int limit. Fixed
682 in version 5.003_04. But always be wary when using large integers.
689 Assignment of return values from numeric equality tests
690 does not work in perl5 when the test evaluates to false (0).
691 Logical tests now return an null, instead of 0
699 Also see L<"General Regular Expression Traps using s///, etc.">
700 for another example of this new feature...
702 =item * Bitwise string ops
704 When bitwise operators which can operate upon either numbers or
705 strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
706 treat the operands as bitstrings so long as the program contained a call
707 to the C<vec()> function. perl5 treats the string operands as bitstrings.
708 (See L<perlop/Bitwise String Operators> for more details.)
712 $betty = $fred & $barney;
714 # Uncomment the next line to change perl4's behavior
715 # ($dummy) = vec("dummy", 0, 0);
723 # If vec() is used anywhere in the program, both print:
728 =head2 General data type traps
730 Perl4-to-Perl5 traps involving most data-types, and their usage
731 within certain expressions and/or context.
737 Negative array subscripts now count from the end of the array.
739 @a = (1, 2, 3, 4, 5);
740 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
742 # perl4 prints: The third element of the array is 4 also expressed as
743 # perl5 prints: The third element of the array is 4 also expressed as 4
747 Setting C<$#array> lower now discards array elements, and makes them
748 impossible to recover.
751 print "Before: ",join('',@a);
753 print ", After: ",join('',@a);
755 print ", Recovered: ",join('',@a),"\n";
757 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
758 # perl5 prints: Before: abcde, After: ab, Recovered: ab
762 Hashes get defined before use
765 die "scalar \$s defined" if defined($s);
766 die "array \@a defined" if defined(@a);
767 die "hash \%h defined" if defined(%h);
770 # perl5 dies: hash %h defined
772 Perl will now generate a warning when it sees defined(@a) and
777 glob assignment from variable to variable will fail if the assigned
778 variable is localized subsequent to the assignment
780 @a = ("This is Perl 4");
785 # perl4 prints: This is Perl 4
790 Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
791 it undefines the associated scalar (but may have other side effects
792 including SEGVs). Perl 5 will also warn if C<undef> is assigned to a
793 typeglob. (Note that assigning C<undef> to a typeglob is different
794 than calling the C<undef> function on a typeglob (C<undef *foo>), which
795 has quite a few effects.
802 # perl4 warns: "Use of uninitialized variable" if using -w
804 # perl5 warns: "Undefined value assigned to typeglob" if using -w
806 =item * (Scalar String)
808 Changes in unary negation (of strings)
809 This change effects both the return value and what it
810 does to auto(magic)increment.
817 # perl4 prints: aab : -0 : 1
818 # perl5 prints: aab : -aab : aac
822 perl 4 lets you modify constants:
826 for ($x = 0; $x < 3; $x++) {
830 print "before: $_[0]";
832 print " after: $_[0]\n";
843 # Modification of a read-only value attempted at foo.pl line 12.
848 The behavior is slightly different for:
850 print "$x", defined $x
853 # perl 5: <no output, $x is not called into existence>
855 =item * (Variable Suicide)
857 Variable suicide behavior is more consistent under Perl 5.
858 Perl5 exhibits the same behavior for hashes and scalars,
859 that perl4 exhibits for only scalars.
861 $aGlobal{ "aKey" } = "global value";
862 print "MAIN:", $aGlobal{"aKey"}, "\n";
867 local( *theArgument ) = @_;
868 local( %aNewLocal ); # perl 4 != 5.001l,m
869 $aNewLocal{"aKey"} = "this should never appear";
870 print "SUB: ", $theArgument{"aKey"}, "\n";
871 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
873 if( $GlobalLevel<4 ) {
888 # SUB: this should never appear
889 # SUB: this should never appear
890 # SUB: this should never appear
894 =head2 Context Traps - scalar, list contexts
898 =item * (list context)
900 The elements of argument lists for formats are now evaluated in list
901 context. This means you can interpolate list values now.
903 @fmt = ("foo","bar","baz");
910 # perl4 errors: Please use commas to separate fields in file
911 # perl5 prints: foo bar baz
913 =item * (scalar context)
915 The C<caller()> function now returns a false value in a scalar context
916 if there is no caller. This lets library files determine if they're
919 caller() ? (print "You rang?\n") : (print "Got a 0\n");
921 # perl4 errors: There is no caller
922 # perl5 prints: Got a 0
924 =item * (scalar context)
926 The comma operator in a scalar context is now guaranteed to give a
927 scalar context to its arguments.
933 # Perl4 prints: x = c # Thinks list context interpolates list
934 # Perl5 prints: x = 3 # Knows scalar uses length of list
936 =item * (list, builtin)
938 C<sprintf()> is prototyped as ($;@), so its first argument is given scalar
939 context. Thus, if passed an array, it will probably not do what you want,
942 @z = ('%s%s', 'foo', 'bar');
946 # perl4 prints: foobar
949 C<printf()> works the same as it did in Perl 4, though:
951 @z = ('%s%s', 'foo', 'bar');
954 # perl4 prints: foobar
955 # perl5 prints: foobar
959 =head2 Precedence Traps
961 Perl4-to-Perl5 traps involving precedence order.
963 Perl 4 has almost the same precedence rules as Perl 5 for the operators
964 that they both have. Perl 4 however, seems to have had some
965 inconsistencies that made the behavior differ from what was documented.
971 LHS vs. RHS of any assignment operator. LHS is evaluated first
972 in perl4, second in perl5; this can affect the relationship
973 between side-effects in sub-expressions.
975 @arr = ( 'left', 'right' );
976 $a{shift @arr} = shift @arr;
977 print join( ' ', keys %a );
980 # perl5 prints: right
984 These are now semantic errors because of precedence:
987 %map = ("a",1,"b",2,"c",3,"d",4);
988 $n = shift @list + 2; # first item in list plus 2
990 $m = keys %map + 2; # number of items in hash plus 2
993 # perl4 prints: n is 3, m is 6
994 # perl5 errors and fails to compile
998 The precedence of assignment operators is now the same as the precedence
999 of assignment. Perl 4 mistakenly gave them the precedence of the associated
1000 operator. So you now must parenthesize them in expressions like
1002 /foo/ ? ($a += 2) : ($a -= 2);
1006 /foo/ ? $a += 2 : $a -= 2
1008 would be erroneously parsed as
1010 (/foo/ ? $a += 2 : $a) -= 2;
1014 $a += /foo/ ? 1 : 2;
1016 now works as a C programmer would expect.
1022 is now incorrect. You need parentheses around the filehandle.
1023 Otherwise, perl5 leaves the statement as its default precedence:
1027 # perl4 opens or dies
1028 # perl5 opens FOO, dying only if 'FOO' is false, i.e. never
1032 perl4 gives the special variable, C<$:> precedence, where perl5
1033 treats C<$::> as main C<package>
1035 $a = "x"; print "$::a";
1037 # perl 4 prints: -:a
1042 perl4 had buggy precedence for the file test operators vis-a-vis
1043 the assignment operators. Thus, although the precedence table
1044 for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1045 C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1046 In perl5, the precedence is as documented.
1050 # perl4 prints: no output
1051 # perl5 prints: Can't modify -e in concatenation
1055 In perl4, keys(), each() and values() were special high-precedence operators
1056 that operated on a single hash, but in perl5, they are regular named unary
1057 operators. As documented, named unary operators have lower precedence
1058 than the arithmetic and concatenation operators C<+ - .>, but the perl4
1059 variants of these operators actually bind tighter than C<+ - .>.
1066 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1068 The perl4 behavior was probably more useful, if less consistent.
1072 =head2 General Regular Expression Traps using s///, etc.
1074 All types of RE traps.
1078 =item * Regular Expression
1080 C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
1081 interpolate $lhs but not $rhs. (And still does not match a literal
1085 $string = '1 2 $a $b';
1086 $string =~ s'$a'$b';
1089 # perl4 prints: $b 2 $a $b
1090 # perl5 prints: 1 2 $a $b
1092 =item * Regular Expression
1094 C<m//g> now attaches its state to the searched string rather than the
1095 regular expression. (Once the scope of a block is left for the sub, the
1096 state of the searched string is lost)
1102 sub doit{local($_) = shift; print "Got $_ "}
1104 # perl4 prints: Got blah Got blah Got blah Got blah
1105 # perl5 prints: infinite loop blah...
1107 =item * Regular Expression
1109 Currently, if you use the C<m//o> qualifier on a regular expression
1110 within an anonymous sub, I<all> closures generated from that anonymous
1111 sub will use the regular expression as it was compiled when it was used
1112 the very first time in any such closure. For instance, if you say
1115 my($left,$right) = @_;
1116 return sub { $_[0] =~ /$left stuff $right/o; };
1118 $good = build_match('foo','bar');
1119 $bad = build_match('baz','blarch');
1120 print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
1121 print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
1122 print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
1124 For most builds of Perl5, this will print:
1129 build_match() will always return a sub which matches the contents of
1130 $left and $right as they were the I<first> time that build_match()
1131 was called, not as they are in the current call.
1133 =item * Regular Expression
1135 If no parentheses are used in a match, Perl4 sets C<$+> to
1136 the whole match, just like C<$&>. Perl5 does not.
1141 # perl4 prints: bcde
1144 =item * Regular Expression
1146 substitution now returns the null string if it fails
1149 $value = ($string =~ s/foo//);
1155 Also see L<Numerical Traps> for another example of this new feature.
1157 =item * Regular Expression
1159 C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1163 $string =~ s`^`hostname`;
1164 print $string, "\n";
1166 # perl4 prints: <the local hostname>
1167 # perl5 prints: hostname
1169 =item * Regular Expression
1171 Stricter parsing of variables used in regular expressions
1173 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1175 # perl4: compiles w/o error
1176 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1178 an added component of this example, apparently from the same script, is
1179 the actual value of the s'd string after the substitution.
1180 C<[$opt]> is a character class in perl4 and an array subscript in perl5
1185 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1189 # perl5 prints: foobar
1191 =item * Regular Expression
1193 Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1194 repeatedly, like C</x/> or C<m!x!>.
1197 sub match { $test =~ m?once?; }
1200 # m?x? matches more then once
1203 # m?x? matches only once
1207 # perl4 prints: perl4
1208 # perl5 prints: perl5
1213 =head2 Subroutine, Signal, Sorting Traps
1215 The general group of Perl4-to-Perl5 traps having to do with
1216 Signals, Sorting, and their related subroutines, as well as
1217 general subroutine traps. Includes some OS-Specific traps.
1223 Barewords that used to look like strings to Perl will now look like subroutine
1224 calls if a subroutine by that name is defined before the compiler sees them.
1226 sub SeeYa { warn"Hasta la vista, baby!" }
1227 $SIG{'TERM'} = SeeYa;
1228 print "SIGTERM is now $SIG{'TERM'}\n";
1230 # perl4 prints: SIGTERM is now main'SeeYa
1231 # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
1233 Use B<-w> to catch this one
1235 =item * (Sort Subroutine)
1237 reverse is no longer allowed as the name of a sort subroutine.
1239 sub reverse{ print "yup "; $a <=> $b }
1240 print sort reverse (2,1,3);
1242 # perl4 prints: yup yup 123
1244 # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()
1246 =item * warn() won't let you specify a filehandle.
1248 Although it _always_ printed to STDERR, warn() would let you specify a
1249 filehandle in perl4. With perl5 it does not.
1253 # perl4 prints: Foo!
1254 # perl5 prints: String found where operator expected
1264 Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1265 within the signal handler function, each time a signal was handled with
1266 perl4. With perl5, the reset is now done correctly. Any code relying
1267 on the handler _not_ being reset will have to be reworked.
1269 Since version 5.002, Perl uses sigaction() under SysV.
1274 $SIG{'INT'} = 'gotit';
1283 while (1) {sleep(10);}
1286 # perl4 (HPUX) prints: Got INT...
1287 # perl5 (HPUX) prints: Got INT... Got INT...
1291 Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
1292 the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
1293 for append, it is impossible to overwrite information already in
1296 open(TEST,">>seek.test");
1297 $start = tell TEST ;
1302 seek(TEST,$start,0);
1303 print TEST "18 characters here";
1305 # perl4 (solaris) seek.test has: 18 characters here
1306 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1312 =head2 Interpolation Traps
1314 Perl4-to-Perl5 traps having to do with how things get interpolated
1315 within certain expressions, statements, contexts, or whatever.
1319 =item * Interpolation
1321 @ now always interpolates an array in double-quotish strings.
1323 print "To: someone@somewhere.com\n";
1325 # perl4 prints: To:someone@somewhere.com
1326 # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
1327 # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
1329 =item * Interpolation
1331 Double-quoted strings may no longer end with an unescaped $ or @.
1335 print "foo is $foo, bar is $bar\n";
1337 # perl4 prints: foo is foo$, bar is bar@
1338 # perl5 errors: Final $ should be \$ or $name
1340 Note: perl5 DOES NOT error on the terminating @ in $bar
1342 =item * Interpolation
1344 Perl now sometimes evaluates arbitrary expressions inside braces that occur
1345 within double quotes (usually when the opening brace is preceded by C<$>
1351 sub foo { return "bar" };
1352 print "|@{w.w.w}|${main'foo}|";
1354 # perl4 prints: |@{w.w.w}|foo|
1355 # perl5 prints: |buz|bar|
1357 Note that you can C<use strict;> to ward off such trappiness under perl5.
1359 =item * Interpolation
1361 The construct "this is $$x" used to interpolate the pid at that point, but
1362 now tries to dereference $x. C<$$> by itself still works fine, however.
1366 print "this is $$x\n";
1368 # perl4 prints: this is XXXx (XXX is the current pid)
1369 # perl5 prints: this is a reference
1371 =item * Interpolation
1373 Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1374 C<$>'s to be protected in the specification of the hash name, or both curlies
1375 to be protected. If both curlies are protected, the result will be compatible
1376 with perl4 and perl5. This is a very common practice, and should be changed
1377 to use the block form of C<eval{}> if possible.
1379 $hashname = "foobar";
1382 eval "\$$hashname{'$key'} = q|$value|";
1383 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1386 # perl5 prints: Nope
1390 eval "\$$hashname{'$key'} = q|$value|";
1394 eval "\$\$hashname{'$key'} = q|$value|";
1396 causes the following result:
1398 # perl4 prints: Nope
1403 eval "\$$hashname\{'$key'\} = q|$value|";
1405 causes the following result:
1409 # and is compatible for both versions
1412 =item * Interpolation
1414 perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1416 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1418 # perl4 prints: This is not perl5
1419 # perl5 prints: This is perl5
1421 =item * Interpolation
1423 You also have to be careful about array references.
1428 perl 5 prints: syntax error
1430 =item * Interpolation
1432 Similarly, watch out for:
1435 print "\$$foo{bar}\n";
1437 # perl4 prints: $baz{bar}
1440 Perl 5 is looking for C<$foo{bar}> which doesn't exist, but perl 4 is
1441 happy just to expand $foo to "baz" by itself. Watch out for this
1442 especially in C<eval>'s.
1444 =item * Interpolation
1446 C<qq()> string passed to C<eval>
1449 foreach \$y (keys %\$x\) {
1454 # perl4 runs this ok
1455 # perl5 prints: Can't find string terminator ")"
1467 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1468 may cause the same script, run under perl5, to fail. The build of perl5
1469 must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1470 to function properly without C<tie>'ing to an extension dbm implementation.
1472 dbmopen (%dbm, "file", undef);
1476 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1481 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1482 may cause the same script, run under perl5, to fail. The error generated
1483 when exceeding the limit on the key/value size will cause perl5 to exit
1486 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1487 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1491 dbm store returned -1, errno 28, key "trap" at - line 3.
1495 dbm store returned -1, errno 28, key "trap" at - line 3.
1499 =head2 Unclassified Traps
1505 =item * C<require>/C<do> trap using returned value
1507 If the file doit.pl has:
1515 And the do.pl file has the following single line:
1519 Running doit.pl gives the following:
1521 # perl 4 prints: 3 (aborts the subroutine early)
1524 Same behavior if you replace C<do> with C<require>.
1526 =item * C<split> on empty string with LIMIT specified
1529 @list = split(/foo/, $string, 2)
1531 Perl4 returns a one element list containing the empty string but Perl5
1532 returns an empty list.
1536 As always, if any of these are ever officially declared as bugs,
1537 they'll be fixed and removed.