Krishna Shamu Sethuraman


perltrap - Perl traps for the unwary


The biggest trap of all is forgetting to use the -w switch; see perlrun. The second biggest trap is not making your entire program runnable under use strict.

Awk Traps

Accustomed awk users should take special note of the following:

  • The English module, loaded via

        use English;

    allows you to refer to special variables (like $RS) as though they were in awk; see perlvar for details.

  • Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.

  • Curly brackets are required on ifs and whiles.

  • Variables begin with "$" or "@" in Perl.

  • Arrays index from 0. Likewise string positions in substr() and index().

  • You have to decide whether your array has numeric or string indices.

  • Associative array values do not spring into existence upon mere reference.

  • You have to decide whether you want to use string or numeric comparisons.

  • Reading an input line does not split it for you. You get to split it yourself to an array. And the split() operator has different arguments.

  • The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is the name of the program executed.) See perlvar.

  • $<digit> does not refer to fields--it refers to substrings matched by the last match pattern.

  • The print() statement does not add field and record separators unless you set $, and $\. You can set $OFS and $ORS if you're using the English module.

  • You must open your files before you print to them.

  • The range operator is "..", not comma. The comma operator works as in C.

  • The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.)

  • The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that awk is basically incompatible with C.)

  • The concatenation operator is ".", not the null string. (Using the null string would render /pat/ /pat/ unparsable, since the third slash would be interpreted as a division operator--the tokener is in fact slightly context sensitive for operators like "/", "?", and ">". And in fact, "." itself can be the beginning of a number.)

  • The next, exit, and continue keywords work differently.

  • The following variables work differently:

          Awk       Perl
          ARGC      $#ARGV or scalar @ARGV
          ARGV[0]   $0
          FILENAME  $ARGV
          FNR       $. - something
          FS        (whatever you like)
          NF        $#Fld, or some such
          NR        $.
          OFMT      $#
          OFS       $,
          ORS       $\
          RLENGTH   length($&)
          RS        $/
          RSTART    length($`)
          SUBSEP    $;
  • You cannot set $RS to a pattern, only a string.

  • When in doubt, run the awk construct through a2p and see what it gives you.

C Traps

Cerebral C programmers should take note of the following:

  • Curly brackets are required on if's and while's.

  • You must use elsif rather than else if.

  • The break and continue keywords from C become in Perl last and next, respectively. Unlike in C, these do NOT work within a do { } while construct.

  • There's no switch statement. (But it's easy to build one on the fly.)

  • Variables begin with "$" or "@" in Perl.

  • printf() does not implement the "*" format for interpolating field widths, but it's trivial to use interpolation of double-quoted strings to achieve the same effect.

  • Comments begin with "#", not "/*".

  • You can't take the address of anything, although a similar operator in Perl 5 is the backslash, which creates a reference.

  • ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0] ends up in $0.

  • System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0.

  • Signal handlers deal with signal names, not numbers. Use kill -l to find their names on your system.

Sed Traps

Seasoned sed programmers should take note of the following:

  • Backreferences in substitutions use "$" rather than "\".

  • The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.

  • The range operator is ..., rather than comma.

Shell Traps

Sharp shell programmers should take note of the following:

  • The backtick operator does variable interpolation without regard to the presence of single quotes in the command.

  • The backtick operator does no translation of the return value, unlike csh.

  • Shells (especially csh) do several levels of substitution on each command line. Perl does substitution only in certain constructs such as double quotes, backticks, angle brackets, and search patterns.

  • Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for BEGIN blocks, which execute at compile time).

  • The arguments are available via @ARGV, not $1, $2, etc.

  • The environment is not automatically made available as separate scalar variables.

Perl Traps

Practicing Perl Programmers should take note of the following:

  • Remember that many operations behave differently in a list context than they do in a scalar one. See perldata for details.

  • Avoid barewords if you can, especially all lower-case ones. You can't tell just by looking at it whether a bareword is a function or a string. By using quotes on strings and parens on function calls, you won't ever get them confused.

  • You cannot discern from mere inspection which built-ins are unary operators (like chop() and chdir()) and which are list operators (like print() and unlink()). (User-defined subroutines can only be list operators, never unary ones.) See perlop.

  • People have a hard time remembering that some functions default to $_, or @ARGV, or whatever, but that others which you might expect to do not.

  • The <FH> construct is not the name of the filehandle, it is a readline operation on that handle. The data read is only assigned to $_ if the file read is the sole condition in a while loop:

        while (<FH>)      { }
        while ($_ = <FH>) { }..
        <FH>;  # data discarded!
  • Remember not to use "=" when you need "=~"; these two constructs are quite different:

        $x =  /foo/;
        $x =~ /foo/;
  • The do {} construct isn't a real loop that you can use loop control on.

  • Use my() for local variables whenever you can get away with it (but see perlform for where you can't). Using local() actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping.

  • If you localize an exported variable in a module, its exported value will not change. The local name becomes an alias to a new value but the external name is still an alias for the original.

Perl4 to Perl5 Traps

Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps.

They're crudely ordered according to the following list:

Discontinuance, Deprecation, and BugFix traps

Anything that's been fixed as a perl4 bug, removed as a perl4 feature or deprecated as a perl4 feature with the intent to encourage usage of some other perl5 feature.

Parsing Traps

Traps that appear to stem from the new parser.

Numerical Traps

Traps having to do with numerical or mathematical operators.

General data type traps

Traps involving perl standard data types.

Context Traps - scalar, list contexts

Traps related to context within lists, scalar statements/declarations.

Precedence Traps

Traps related to the precedence of parsing, evaluation, and execution of code.

General Regular Expression Traps using s///, etc.

Traps related to the use of pattern matching.

Subroutine, Signal, Sorting Traps

Traps related to the use of signals and signal handlers, general subroutines, and sorting, along with sorting subroutines.

OS Traps

OS-specific traps.

DBM Traps

Traps specific to the use of dbmopen(), and specific dbm implementations.

Unclassified Traps

Everything else.

If you find an example of a conversion trap that is not listed here, please submit it to Bill Middleton for inclusion. Also note that at least some of these can be caught with -w.

Discontinuance, Deprecation, and BugFix traps

Anything that has been discontinued, deprecated, or fixed as a bug from perl4.

  • Discontinuance

    Symbols starting with "_" are no longer forced into package main, except for $_ itself (and @_, etc.).

        package test;
        $_legacy = 1;
        package main;
        print "\$_legacy is ",$_legacy,"\n";
        # perl4 prints: $_legacy is 1
        # perl5 prints: $_legacy is
  • Deprecation

    Double-colon is now a valid package separator in a variable name. Thus these behave differently in perl4 vs. perl5, since the packages don't exist.

        print "$a::$b::$c ";
        print "$var::abc::xyz\n";
        # perl4 prints: 1::2::3 4::abc::xyz
        # perl5 prints: 3

    Given that :: is now the preferred package delimiter, it is debatable whether this should be classed as a bug or not. (The older package delimiter, ' ,is used here)

        $x = 10 ;
        print "x=${'x}\n" ;
        # perl4 prints: x=10
        # perl5 prints: Can't find string terminator "'" anywhere before EOF

    Also see precedence traps, for parsing $:.

  • BugFix

    The second and third arguments of splice() are now evaluated in scalar context (as the Camel says) rather than list context.

        sub sub1{return(0,2) }          # return a 2-elem array
        sub sub2{ return(1,2,3)}        # return a 3-elem array
        @a1 = ("a","b","c","d","e"); 
        @a2 = splice(@a1,&sub1,&sub2);
        print join(' ',@a2),"\n";
        # perl4 prints: a b
        # perl5 prints: c d e 
  • Discontinuance

    You can't do a goto into a block that is optimized away. Darn.

        goto marker1;
            print "Here I is!\n";
        # perl4 prints: Here I is!
        # perl5 dumps core (SEGV)
  • Discontinuance

    It is no longer syntactically legal to use whitespace as the name of a variable, or as a delimiter for any kind of quote construct. Double darn.

        $a = ("foo bar");
        $b = q baz ;
        print "a is $a, b is $b\n";
        # perl4 prints: a is foo bar, b is baz
        # perl5 errors: Bare word found where operator expected
  • Discontinuance

    The archaic while/if BLOCK BLOCK syntax is no longer supported.

        if { 1 } {
            print "True!";
        else {
            print "False!";
        # perl4 prints: True!
        # perl5 errors: syntax error at line 1, near "if {"
  • BugFix

    The ** operator now binds more tightly than unary minus. It was documented to work this way before, but didn't.

        print -4**2,"\n";
        # perl4 prints: 16
        # perl5 prints: -16
  • Discontinuance

    The meaning of foreach{} has changed slightly when it is iterating over a list which is not an array. This used to assign the list to a temporary array, but no longer does so (for efficiency). This means that you'll now be iterating over the actual values, not over copies of the values. Modifications to the loop variable can change the original values.

        @list = ('ab','abc','bcd','def');
        foreach $var (grep(/ab/,@list)){
            $var = 1;
        print (join(':',@list));
        # perl4 prints: ab:abc:bcd:def
        # perl5 prints: 1:1:bcd:def

    To retain Perl4 semantics you need to assign your list explicitly to a temporary array and then iterate over that. For example, you might need to change

        foreach $var (grep(/ab/,@list)){


        foreach $var (@tmp = grep(/ab/,@list)){

    Otherwise changing $var will clobber the values of @list. (This most often happens when you use $_ for the loop variable, and call subroutines in the loop that don't properly localize $_.)

  • Discontinuance

    split with no arguments now behaves like split ' ' (which doesn't return an initial null field if $_ starts with whitespace), it used to behave like split /\s+/ (which does).

        $_ = ' hi mom';
        print join(':', split);
        # perl4 prints: :hi:mom
        # perl5 prints: hi:mom
  • Deprecation

    Some error messages will be different.

  • Discontinuance

    Some bugs may have been inadvertently removed. :-)

Parsing Traps

Perl4-to-Perl5 traps from having to do with parsing.

  • Parsing

    Note the space between . and =

        $string . = "more string";
        print $string;
        # perl4 prints: more string
        # perl5 prints: syntax error at - line 1, near ". ="
  • Parsing

    Better parsing in perl 5

        sub foo {}
        print("hello, world\n");
        # perl4 prints: hello, world
        # perl5 prints: syntax error
  • Parsing

    "if it looks like a function, it is a function" rule.

        ($foo == 1) ? "is one\n" : "is zero\n";
        # perl4 prints: is zero
        # perl5 warns: "Useless use of a constant in void context" if using -w

Numerical Traps

Perl4-to-Perl5 traps having to do with numerical operators, operands, or output from same.

  • Numerical

    Formatted output and significant digits

        print 7.373504 - 0, "\n";
        printf "%20.18f\n", 7.373504 - 0; 
        # Perl4 prints:
        # Perl5 prints:
  • Numerical

    This specific item has been deleted. It demonstrated how the autoincrement operator would not catch when a number went over the signed int limit. Fixed in 5.003_04. But always be wary when using large ints. If in doubt:

       use Math::BigInt;
  • Numerical

    Assignment of return values from numeric equality tests does not work in perl5 when the test evaluates to false (0). Logical tests now return an null, instead of 0

        $p = ($test == 1);
        print $p,"\n";
        # perl4 prints: 0
        # perl5 prints:

    Also see the "General Regular Expression Traps" tests for another example of this new feature...

General data type traps

Perl4-to-Perl5 traps involving most data-types, and their usage within certain expressions and/or context.

  • (Arrays)

    Negative array subscripts now count from the end of the array.

        @a = (1, 2, 3, 4, 5);
        print "The third element of the array is $a[3] also expressed as $a[-2] \n";
        # perl4 prints: The third element of the array is 4 also expressed as
        # perl5 prints: The third element of the array is 4 also expressed as 4
  • (Arrays)

    Setting $#array lower now discards array elements, and makes them impossible to recover.

        @a = (a,b,c,d,e); 
        print "Before: ",join('',@a);
        $#a =1; 
        print ", After: ",join('',@a);
        $#a =3;
        print ", Recovered: ",join('',@a),"\n";
        # perl4 prints: Before: abcde, After: ab, Recovered: abcd
        # perl5 prints: Before: abcde, After: ab, Recovered: ab
  • (Hashes)

    Hashes get defined before use

        die "scalar \$s defined" if defined($s);
        die "array \@a defined" if defined(@a);
        die "hash \%h defined" if defined(%h);
        # perl4 prints:
        # perl5 dies: hash %h defined
  • (Globs)

    glob assignment from variable to variable will fail if the assigned variable is localized subsequent to the assignment

        @a = ("This is Perl 4");
        *b = *a;
        print @b,"\n";
        # perl4 prints: This is Perl 4
        # perl5 prints:
        # Another example
        *fred = *barney; # fred is aliased to barney
        @barney = (1, 2, 4);
        # @fred;
        print "@fred";  # should print "1, 2, 4"
        # perl4 prints: 1 2 4
        # perl5 prints: Literal @fred now requires backslash 
  • (Scalar String)

    Changes in unary negation (of strings) This change effects both the return value and what it does to auto(magic)increment.

        $x = "aaa";
        print ++$x," : ";
        print -$x," : ";
        print ++$x,"\n";
        # perl4 prints: aab : -0 : 1
        # perl5 prints: aab : -aab : aac
  • (Constants)

    perl 4 lets you modify constants:

        $foo = "x";
        for ($x = 0; $x < 3; $x++) {
        sub mod {
            print "before: $_[0]";
            $_[0] = "m";
            print "  after: $_[0]\n";
        # perl4:
        # before: x  after: m
        # before: a  after: m
        # before: m  after: m
        # before: m  after: m
        # Perl5:
        # before: x  after: m
        # Modification of a read-only value attempted at line 12.
        # before: a
  • (Scalars)

    The behavior is slightly different for:

        print "$x", defined $x
        # perl 4: 1
        # perl 5: <no output, $x is not called into existence>
  • (Variable Suicide)

    Variable suicide behavior is more consistent under Perl 5. Perl5 exhibits the same behavior for associative arrays and scalars, that perl4 exhibits only for scalars.

        $aGlobal{ "aKey" } = "global value";
        print "MAIN:", $aGlobal{"aKey"}, "\n";
        $GlobalLevel = 0;
        &test( *aGlobal );
        sub test {
            local( *theArgument ) = @_;
            local( %aNewLocal ); # perl 4 != 5.001l,m
            $aNewLocal{"aKey"} = "this should never appear";  
            print "SUB: ", $theArgument{"aKey"}, "\n";
            $aNewLocal{"aKey"} = "level $GlobalLevel";   # what should print
            if( $GlobalLevel<4 ) {
                &test( *aNewLocal );
        # Perl4:
        # MAIN:global value
        # SUB: global value
        # SUB: level 0
        # SUB: level 1
        # SUB: level 2
        # Perl5:
        # MAIN:global value
        # SUB: global value
        # SUB: this should never appear
        # SUB: this should never appear
        # SUB: this should never appear

Context Traps - scalar, list contexts

  • (list context)

    The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now.

        @fmt = ("foo","bar","baz");
        format STDOUT=
        @<<<<< @||||| @>>>>>
        # perl4 errors:  Please use commas to separate fields in file
        # perl5 prints: foo     bar      baz
  • (scalar context)

    The caller() function now returns a false value in a scalar context if there is no caller. This lets library files determine if they're being required.

        caller() ? (print "You rang?\n") : (print "Got a 0\n");
        # perl4 errors: There is no caller
        # perl5 prints: Got a 0
  • (scalar context)

    The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments.

        @y= ('a','b','c');
        $x = (1, 2, @y);
        print "x = $x\n";
        # Perl4 prints:  x = c   # Thinks list context interpolates list
        # Perl5 prints:  x = 3   # Knows scalar uses length of list
  • (list, builtin)

    sprintf() funkiness (array argument converted to scalar array count) This test could be added to t/op/sprintf.t

        @z = ('%s%s', 'foo', 'bar');
        $x = sprintf(@z);
        if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
        # perl4 prints: ok 2
        # perl5 prints: not ok 2

    printf() works fine, though:

        printf STDOUT (@z);
        print "\n"; 
        # perl4 prints: foobar
        # perl5 prints: foobar

    Probably a bug.

Precedence Traps

Perl4-to-Perl5 traps involving precedence order.

  • Precedence

    LHS vs. RHS when both sides are getting an op.

        @arr = ( 'left', 'right' );
        $a{shift @arr} = shift @arr;
        print join( ' ', keys %a );
        # perl4 prints: left
        # perl5 prints: right
  • Precedence

    These are now semantic errors because of precedence:

        @list = (1,2,3,4,5);
        %map = ("a",1,"b",2,"c",3,"d",4);
        $n = shift @list + 2;   # first item in list plus 2
        print "n is $n, ";
        $m = keys %map + 2;     # number of items in hash plus 2
        print "m is $m\n";
        # perl4 prints: n is 3, m is 6
        # perl5 errors and fails to compile
  • Precedence

    The precedence of assignment operators is now the same as the precedence of assignment. Perl 4 mistakenly gave them the precedence of the associated operator. So you now must parenthesize them in expressions like

        /foo/ ? ($a += 2) : ($a -= 2);


        /foo/ ? $a += 2 : $a -= 2

    would be erroneously parsed as

        (/foo/ ? $a += 2 : $a) -= 2;

    On the other hand,

        $a += /foo/ ? 1 : 2; 

    now works as a C programmer would expect.

  • Precedence

        open FOO || die;

    is now incorrect. You need parens around the filehandle. Otherwise, perl5 leaves the statement as it's default precedence:

        open(FOO || die);
        # perl4 opens or dies
        # perl5 errors: Precedence problem: open FOO should be open(FOO)
  • Precedence

    perl4 gives the special variable, $: precedence, where perl5 treats $:: as main package

        $a = "x"; print "$::a";
        # perl 4 prints: -:a
        # perl 5 prints: x
  • Precedence

    concatenation precedence over filetest operator?

        -e $foo .= "q" 
        # perl4 prints: no output
        # perl5 prints: Can't modify -e in concatenation
  • Precedence

    Assignment to value takes precedence over assignment to key in perl5 when using the shift operator on both sides.

        @arr = ( 'left', 'right' );
        $a{shift @arr} = shift @arr;
        print join( ' ', keys %a );
        # perl4 prints: left
        # perl5 prints: right

General Regular Expression Traps using s///, etc.

All types of RE traps.

  • Regular Expression

    s'$lhs'$rhs' now does no interpolation on either side. It used to interpolate $lhs but not $rhs. (And still does not match a literal '$' in string)

        $string = '1 2 $a $b';
        $string =~ s'$a'$b';
        print $string,"\n";
        # perl4 prints: $b 2 $a $b
        # perl5 prints: 1 2 $a $b
  • Regular Expression

    m//g now attaches its state to the searched string rather than the regular expression. (Once the scope of a block is left for the sub, the state of the searched string is lost)

        $_ = "ababab";
        sub doit{local($_) = shift; print "Got $_ "}
        # perl4 prints: blah blah blah
        # perl5 prints: infinite loop blah...
  • Regular Expression

    If no parentheses are used in a match, Perl4 sets $+ to the whole match, just like $&. Perl5 does not.

        "abcdef" =~ /b.*e/;
        print "\$+ = $+\n";
        # perl4 prints: bcde
        # perl5 prints:
  • Regular Expression

    substitution now returns the null string if it fails

        $string = "test";
        $value = ($string =~ s/foo//);
        print $value, "\n";
        # perl4 prints: 0
        # perl5 prints:

    Also see "Numerical Traps" for another example of this new feature.

  • Regular Expression

    s`lhs`rhs` (using backticks) is now a normal substitution, with no backtick expansion

        $string = "";
        $string =~ s`^`hostname`;
        print $string, "\n";
        # perl4 prints: <the local hostname>
        # perl5 prints: hostname
  • Regular Expression

    Stricter parsing of variables used in regular expressions

        # perl4: compiles w/o error
        # perl5: with Scalar found where operator expected ..., near "$opt$plus"

    an added component of this example, apparently from the same script, is the actual value of the s'd string after the substitution. [$opt] is a character class in perl4 and an array subscript in perl5

        $grpc = 'a'; 
        $opt  = 'r';
        $_ = 'bar';
        print ;
        # perl4 prints: foo
        # perl5 prints: foobar
  • Regular Expression

    Under perl5, m?x? matches only once, like ?x?. Under perl4, it matched repeatedly, like /x/ or m!x!.

        $test = "once";
        sub match { $test =~ m?once?; }
        if( &match() ) {
            # m?x? matches more then once
            print "perl4\n";
        } else { 
            # m?x? matches only once
            print "perl5\n"; 
        # perl4 prints: perl4
        # perl5 prints: perl5

Subroutine, Signal, Sorting Traps

The general group of Perl4-to-Perl5 traps having to do with Signals, Sorting, and their related subroutines, as well as general subroutine traps. Includes some OS-Specific traps.

  • (Signals)

    Barewords that used to look like strings to Perl will now look like subroutine calls if a subroutine by that name is defined before the compiler sees them.

        sub SeeYa { warn"Hasta la vista, baby!" }
        $SIG{'TERM'} = SeeYa;
        print "SIGTERM is now $SIG{'TERM'}\n";
        # perl4 prints: SIGTERM is main'SeeYa
        # perl5 prints: SIGTERM is now main::1

    Use -w to catch this one

  • (Sort Subroutine)

    reverse is no longer allowed as the name of a sort subroutine.

        sub reverse{ print "yup "; $a <=> $b }
        print sort reverse a,b,c;  
        # perl4 prints: yup yup yup yup abc
        # perl5 prints: abc 
  • warn() won't let you specify a filehandle.

    Although it _always_ printed to STDERR, warn() would let you specify a filehandle in perl4. With perl5 it does not.

        warn STDERR "Foo!";
        # perl4 prints: Foo!
        # perl5 prints: String found where operator expected 

OS Traps

  • (SysV)

    Under HPUX, and some other SysV OS's, one had to reset any signal handler, within the signal handler function, each time a signal was handled with perl4. With perl5, the reset is now done correctly. Any code relying on the handler _not_ being reset will have to be reworked.

    5.002 and beyond uses sigaction() under SysV

        sub gotit {
            print "Got @_... "; 
        $SIG{'INT'} = 'gotit';
        $| = 1;
        $pid = fork;
        if ($pid) {
            kill('INT', $pid);
            kill('INT', $pid);
        } else { 
            while (1) {sleep(10);}
        # perl4 (HPUX) prints: Got INT...
        # perl5 (HPUX) prints: Got INT... Got INT...
  • (SysV)

    Under SysV OS's, seek() on a file opened to append >> now does the right thing w.r.t. the fopen() man page. e.g. - When a file is opened for append, it is impossible to overwrite information already in the file.

        $start = tell TEST ;  
        foreach(1 .. 9){
            print TEST "$_ ";
        $end = tell TEST ;
        print TEST "18 characters here";
        # perl4 (solaris) seek.test has: 18 characters here
        # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here

Interpolation Traps

Perl4-to-Perl5 traps having to do with how things get interpolated within certain expressions, statements, contexts, or whatever.

  • Interpolation

    @ now always interpolates an array in double-quotish strings.

        print "To:\n"; 
        # perl4 prints:
        # perl5 errors : Literal @somewhere now requires backslash
  • Interpolation

    Double-quoted strings may no longer end with an unescaped $ or @.

        $foo = "foo$";
        $bar = "bar@";
        print "foo is $foo, bar is $bar\n";
        # perl4 prints: foo is foo$, bar is bar@
        # perl5 errors: Final $ should be \$ or $name

    Note: perl5 DOES NOT error on the terminating @ in $bar

  • Interpolation

    Perl now sometimes evaluates arbitrary expressions inside braces that occur within double quotes (usually when the opening brace is preceded by $ or @).

        @www = "buz";
        $foo = "foo";
        $bar = "bar";
        sub foo { return "bar" };
        print "|@{w.w.w}|${main'foo}|";
        # perl4 prints: |@{w.w.w}|foo|
        # perl5 prints: |buz|bar|

    Note that you can use strict; to ward off such trappiness under perl5.

  • Interpolation

    The construct "this is $$x" used to interpolate the pid at that point, but now apparently tries to dereference $x. $$ by itself still works fine, however.

        print "this is $$x\n";
        # perl4 prints: this is XXXx   (XXX is the current pid)
        # perl5 prints: this is
  • Interpolation

    Creation of hashes on the fly with eval "EXPR" now requires either both $'s to be protected in the specification of the hash name, or both curlies to be protected. If both curlies are protected, the result will be compatible with perl4 and perl5. This is a very common practice, and should be changed to use the block form of eval{} if possible.

        $hashname = "foobar";
        $key = "baz";
        $value = 1234;
        eval "\$$hashname{'$key'} = q|$value|";
        (defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");
        # perl4 prints: Yup
        # perl5 prints: Nope


        eval "\$$hashname{'$key'} = q|$value|";


        eval "\$\$hashname{'$key'} = q|$value|";

    causes the following result:

        # perl4 prints: Nope
        # perl5 prints: Yup

    or, changing to

        eval "\$$hashname\{'$key'\} = q|$value|";

    causes the following result:

        # perl4 prints: Yup
        # perl5 prints: Yup
        # and is compatible for both versions
  • Interpolation

    perl4 programs which unconsciously rely on the bugs in earlier perl versions.

        perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
        # perl4 prints: This is not perl5
        # perl5 prints: This is perl5
  • Interpolation

    You also have to be careful about array references.

        print "$foo{"
        perl 4 prints: {
        perl 5 prints: syntax error
  • Interpolation

    Similarly, watch out for:

        $foo = "array";
        print "\$$foo{bar}\n";
        # perl4 prints: $array{bar}
        # perl5 prints: $

    Perl 5 is looking for $array{bar} which doesn't exist, but perl 4 is happy just to expand $foo to "array" by itself. Watch out for this especially in eval's.

  • Interpolation

    qq() string passed to eval

        eval qq(
            foreach \$y (keys %\$x\) {
        # perl4 runs this ok
        # perl5 prints: Can't find string terminator ")" 

DBM Traps

General DBM traps.

  • DBM

    Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the same script, run under perl5, to fail. The build of perl5 must have been linked with the same dbm/ndbm as the default for dbmopen() to function properly without tie'ing to an extension dbm implementation.

        dbmopen (%dbm, "file", undef);
        print "ok\n";
        # perl4 prints: ok
        # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
  • DBM

    Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the same script, run under perl5, to fail. The error generated when exceeding the limit on the key/value size will cause perl5 to exit immediately.

        dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
        $DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm
        print "YUP\n";
        # perl4 prints:
        dbm store returned -1, errno 28, key "trap" at - line 3.
        # perl5 prints:
        dbm store returned -1, errno 28, key "trap" at - line 3.

Unclassified Traps

Everything else.

  • Unclassified

    require/do trap using returned value

    If the file has:

        sub foo {
            $rc = do "./";
            return 8;
        print &foo, "\n";

    And the file has the following single line:

        return 3;

    Running gives the following:

        # perl 4 prints: 3 (aborts the subroutine early)
        # perl 5 prints: 8 

    Same behavior if you replace do with require.

As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.