MirOS Manual: perltrap(1)


PERLTRAP(1)     Perl Programmers Reference Guide      PERLTRAP(1)

NAME

     perltrap - Perl traps for the unwary

DESCRIPTION

     The biggest trap of all is forgetting to "use warnings" or
     use the -w switch; see perllexwarn and perlrun. The second
     biggest trap is not making your entire program runnable
     under "use strict".  The third biggest trap is not reading
     the list of changes in this version of Perl; see perldelta.

     Awk Traps

     Accustomed awk users should take special note of the follow-
     ing:

     +   A Perl program executes only once, not once for each
         input line.  You can do an implicit loop with "-n" or
         "-p".

     +   The English module, loaded via

             use English;

         allows you to refer to special variables (like $/) with
         names (like $RS), as though they were in awk; see perl-
         var 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 "if"s and "while"s.

     +   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.

     +   Hash 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 to an array yourself.  And the split()
         operator has different arguments than awk's.

     +   The current input line is normally in $_, not $0.  It

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         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 sub-
         strings 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 opera-
         tor 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, because the third slash would be interpreted
         as a division operator--the tokenizer 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 dif-
         ferently.

     +   The following variables work differently:

               Awk       Perl
               ARGC      scalar @ARGV (compare with $#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.

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

     C/C++ Traps

     Cerebral C and C++ programmers should take note of the fol-
     lowing:

     +   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.
         See "Loop Control" in perlsyn.

     +   There's no switch statement.  (But it's easy to build
         one on the fly, see "Basic BLOCKs and Switch Statements"
         in perlsyn)

     +   Variables begin with "$", "@" or "%" in Perl.

     +   Comments begin with "#", not "/*" or "//".  Perl may
         interpret C/C++ comments as division operators, unter-
         minated regular expressions or the defined-or operator.

     +   You can't take the address of anything, although a simi-
         lar operator in Perl 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. (system(), however,
         returns zero for success.)

     +   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:

     +   A Perl program executes only once, not once for each
         input line.  You can do an implicit loop with "-n" or
         "-p".

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

     +   The pattern matching metacharacters "(", ")", and "|" do

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         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 substitu-
         tion on each command line.  Perl does substitution in
         only certain constructs such as double quotes, back-
         ticks, 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.

     +   The shell's "test" uses "=", "!=", "<" etc for string
         comparisons and "-eq", "-ne", "-lt" etc for numeric com-
         parisons. This is the reverse of Perl, which uses "eq",
         "ne", "lt" for string comparisons, and "==", "!=" "<"
         etc for numeric comparisons.

     Perl Traps

     Practicing Perl Programmers should take note of the follow-
     ing:

     +   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 lowercase
         ones. You can't tell by just looking at it whether a
         bareword is a function or a string.  By using quotes on
         strings and parentheses on function calls, you won't
         ever get them confused.

     +   You cannot discern from mere inspection which builtins
         are unary operators (like chop() and chdir()) and which

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         are list operators (like print() and unlink()). (Unless
         prototyped, user-defined subroutines can only be list
         operators, never unary ones.)  See perlop and perlsub.

     +   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 assigned to $_ only if the file read is the sole con-
         dition in a while loop:

             while (<FH>)      { }
             while (defined($_ = <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 vari-
         able, 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 follow-
     ing 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.

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     Numerical Traps
         Traps having to do with numerical or mathematical opera-
         tors.

     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 <perlbug@perl.org> for
     inclusion. Also note that at least some of these can be
     caught with the "use warnings" pragma or the -w switch.

     Discontinuance, Deprecation, and BugFix traps

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

     * Symbols starting with "_" no longer forced into main
         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";

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             # perl4 prints: $_legacy is 1
             # perl5 prints: $_legacy is

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

             $a=1;$b=2;$c=3;$var=4;
             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

         You can avoid this problem, and remain compatible with
         perl4, if you always explicitly include the package
         name:

             $x = 10;
             print "x=${main'x}\n";

         Also see precedence traps, for parsing $:.

     * 2nd and 3rd args to "splice()" are now in scalar context
         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-element list
             sub sub2{ return(1,2,3)}        # return a 3-element list
             @a1 = ("a","b","c","d","e");
             @a2 = splice(@a1,&sub1,&sub2);
             print join(' ',@a2),"\n";

             # perl4 prints: a b
             # perl5 prints: c d e

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

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             goto marker1;

             for(1){
             marker1:
                 print "Here I is!\n";
             }

             # perl4 prints: Here I is!
             # perl5 errors: Can't "goto" into the middle of a foreach loop

     * Can't use whitespace as variable name or quote delimiter
         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: Bareword found where operator expected

     * "while/if BLOCK BLOCK" gone
         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 test.pl line 1, near "if {"

     * "**" binds tighter than unary minus
         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

     * "foreach" changed when iterating over a list
         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

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         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)){

         to

             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 $_.)

     * "split" with no args behavior changed
         "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

     * -e behavior fixed
         Perl 4 would ignore any text which was attached to an -e
         switch, always taking the code snippet from the follow-
         ing arg.  Additionally, it would silently accept an -e
         switch without a following arg.  Both of these behaviors
         have been fixed.

             perl -e'print "attached to -e"' 'print "separate arg"'

             # perl4 prints: separate arg
             # perl5 prints: attached to -e

             perl -e

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             # perl4 prints:
             # perl5 dies: No code specified for -e.

     * "push" returns number of elements in resulting list
         In Perl 4 the return value of "push" was undocumented,
         but it was actually the last value being pushed onto the
         target list.  In Perl 5 the return value of "push" is
         documented, but has changed, it is the number of ele-
         ments in the resulting list.

             @x = ('existing');
             print push(@x, 'first new', 'second new');

             # perl4 prints: second new
             # perl5 prints: 3

     * Some error messages differ
         Some error messages will be different.

     * "split()" honors subroutine args
         In Perl 4, if in list context the delimiters to the
         first argument of "split()" were "??", the result would
         be placed in @_ as well as being returned.   Perl 5 has
         more respect for your subroutine arguments.

     * Bugs removed
         Some bugs may have been inadvertently removed.  :-)

     Parsing Traps

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

     * Space between . and = triggers syntax error
         Note the space between . and =

             $string . = "more string";
             print $string;

             # perl4 prints: more string
             # perl5 prints: syntax error at - line 1, near ". ="

     * Better parsing in perl 5
         Better parsing in perl 5

             sub foo {}
             &foo
             print("hello, world\n");

             # perl4 prints: hello, world
             # perl5 prints: syntax error

     * Function parsing

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

           print
             ($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

     * String interpolation of $#array differs
         String interpolation of the $#array construct differs
         when braces are to used around the name.

             @a = (1..3);
             print "${#a}";

             # perl4 prints: 2
             # perl5 fails with syntax error

             @ = (1..3);
             print "$#{a}";

             # perl4 prints: {a}
             # perl5 prints: 2

     * Perl guesses on "map", "grep" followed by "{" if it starts
      BLOCK or hash ref
         When perl sees "map {" (or "grep {"), it has to guess
         whether the "{" starts a BLOCK or a hash reference. If
         it guesses wrong, it will report a syntax error near the
         "}" and the missing (or unexpected) comma.

         Use unary "+" before "{" on a hash reference, and unary
         "+" applied to the first thing in a BLOCK (after "{"),
         for perl to guess right all the time. (See "map" in
         perlfunc.)

     Numerical Traps

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

     * Formatted output and significant digits
          Formatted output and significant digits.  In general,
          Perl 5 tries to be more precise.  For example, on a
          Solaris Sparc:

              print 7.373504 - 0, "\n";
              printf "%20.18f\n", 7.373504 - 0;

              # Perl4 prints:
              7.3750399999999996141
              7.375039999999999614

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              # Perl5 prints:
              7.373504
              7.375039999999999614

          Notice how the first result looks better in Perl 5.

          Your results may vary, since your floating point for-
          matting routines and even floating point format may be
          slightly different.

     * Auto-increment operator over signed int limit deleted
          This specific item has been deleted.  It demonstrated
          how the auto-increment operator would not catch when a
          number went over the signed int limit.  Fixed in ver-
          sion 5.003_04.  But always be wary when using large
          integers. If in doubt:

             use Math::BigInt;

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

              $p = ($test == 1);
              print $p,"\n";

              # perl4 prints: 0
              # perl5 prints:

          Also see "General Regular Expression Traps using s///,
          etc." for another example of this new feature...

     * Bitwise string ops
          When bitwise operators which can operate upon either
          numbers or strings ("& | ^ ~") are given only strings
          as arguments, perl4 would treat the operands as bit-
          strings so long as the program contained a call to the
          "vec()" function. perl5 treats the string operands as
          bitstrings. (See "Bitwise String Operators" in perlop
          for more details.)

              $fred = "10";
              $barney = "12";
              $betty = $fred & $barney;
              print "$betty\n";
              # Uncomment the next line to change perl4's behavior
              # ($dummy) = vec("dummy", 0, 0);

              # Perl4 prints:
              8

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              # Perl5 prints:
              10

              # If vec() is used anywhere in the program, both print:
              10

     General data type traps

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

     * Negative array subscripts now count from the end of array
          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

     * Setting $#array lower now discards array elements
          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 get defined before use
          Hashes get defined before use

              local($s,@a,%h);
              die "scalar \$s defined" if defined($s);
              die "array \@a defined" if defined(@a);
              die "hash \%h defined" if defined(%h);

              # perl4 prints:
              # perl5 dies: hash %h defined

          Perl will now generate a warning when it sees
          defined(@a) and defined(%h).

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

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          assignment

              @a = ("This is Perl 4");
              *b = *a;
              local(@a);
              print @b,"\n";

              # perl4 prints: This is Perl 4
              # perl5 prints:

     * Assigning "undef" to glob
          Assigning "undef" to a glob has no effect in Perl 5.
          In Perl 4 it undefines the associated scalar (but may
          have other side effects including SEGVs). Perl 5 will
          also warn if "undef" is assigned to a typeglob. (Note
          that assigning "undef" to a typeglob is different than
          calling the "undef" function on a typeglob ("undef
          *foo"), which has quite a few effects.

              $foo = "bar";
              *foo = undef;
              print $foo;

              # perl4 prints:
              # perl4 warns: "Use of uninitialized variable" if using -w
              # perl5 prints: bar
              # perl5 warns: "Undefined value assigned to typeglob" if using -w

     * Changes in unary negation (of strings)
          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

     * Modifying of constants prohibited
          perl 4 lets you modify constants:

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              $foo = "x";
              &mod($foo);
              for ($x = 0; $x < 3; $x++) {
                  &mod("a");
              }
              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 foo.pl line 12.
              # before: a

     * "defined $var" behavior changed
          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 hashes and
          scalars, that perl4 exhibits for only 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
                  $GlobalLevel++;
                  if( $GlobalLevel<4 ) {
                      &test( *aNewLocal );
                  }
              }

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              # 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

     * Elements of argument lists for formats evaluated in list con-
      text
          The elements of argument lists for formats are now
          evaluated in list context.  This means you can interpo-
          late list values now.

              @fmt = ("foo","bar","baz");
              format STDOUT=
              @<<<<< @||||| @>>>>>
              @fmt;
              .
              write;

              # perl4 errors:  Please use commas to separate fields in file
              # perl5 prints: foo     bar      baz

     * "caller()" returns false value in scalar context if no caller
      present
          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

     * Comma operator in scalar context gives scalar context to args
          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";

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              # Perl4 prints:  x = c   # Thinks list context interpolates list
              # Perl5 prints:  x = 3   # Knows scalar uses length of list

     * "sprintf()" prototyped as "($;@)"
          "sprintf()" is prototyped as ($;@), so its first argu-
          ment is given scalar context. Thus, if passed an array,
          it will probably not do what you want, unlike Perl 4:

              @z = ('%s%s', 'foo', 'bar');
              $x = sprintf(@z);
              print $x;

              # perl4 prints: foobar
              # perl5 prints: 3

          "printf()" works the same as it did in Perl 4, though:

              @z = ('%s%s', 'foo', 'bar');
              printf STDOUT (@z);

              # perl4 prints: foobar
              # perl5 prints: foobar

     Precedence Traps

     Perl4-to-Perl5 traps involving precedence order.

     Perl 4 has almost the same precedence rules as Perl 5 for
     the operators that they both have.  Perl 4 however, seems to
     have had some inconsistencies that made the behavior differ
     from what was documented.

     * LHS vs. RHS of any assignment operator
          LHS vs. RHS of any assignment operator.  LHS is
          evaluated first in perl4, second in perl5; this can
          affect the relationship between side-effects in
          sub-expressions.

              @arr = ( 'left', 'right' );
              $a{shift @arr} = shift @arr;
              print join( ' ', keys %a );

              # perl4 prints: left
              # perl5 prints: right

     * Semantic errors introduced due to precedence
          These are now semantic errors because of precedence:

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              @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 of assignment operators same as the precedence of
      assignment
          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);

          Otherwise

              /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.

     * "open" requires parentheses around filehandle
              open FOO || die;

          is now incorrect.  You need parentheses around the
          filehandle. Otherwise, perl5 leaves the statement as
          its default precedence:

              open(FOO || die);

              # perl4 opens or dies
              # perl5 opens FOO, dying only if 'FOO' is false, i.e. never

     * $: precedence over $:: gone
          perl4 gives the special variable, $: precedence, where
          perl5 treats $:: as main "package"

              $a = "x"; print "$::a";

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              # perl 4 prints: -:a
              # perl 5 prints: x

     * Precedence of file test operators documented
          perl4 had buggy precedence for the file test operators
          vis-a-vis the assignment operators.  Thus, although the
          precedence table for perl4 leads one to believe "-e
          $foo .= "q"" should parse as "((-e $foo) .= "q")", it
          actually parses as "(-e ($foo .= "q"))". In perl5, the
          precedence is as documented.

              -e $foo .= "q"

              # perl4 prints: no output
              # perl5 prints: Can't modify -e in concatenation

     * "keys", "each", "values" are regular named unary operators
          In perl4, keys(), each() and values() were special
          high-precedence operators that operated on a single
          hash, but in perl5, they are regular named unary opera-
          tors.  As documented, named unary operators have lower
          precedence than the arithmetic and concatenation opera-
          tors "+ - .", but the perl4 variants of these operators
          actually bind tighter than "+ - .". Thus, for:

              %foo = 1..10;
              print keys %foo - 1

              # perl4 prints: 4
              # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)

          The perl4 behavior was probably more useful, if less
          consistent.

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

     All types of RE traps.

     * "s'$lhs'$rhs'" interpolates on either side
          "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)

              $a=1;$b=2;
              $string = '1 2 $a $b';
              $string =~ s'$a'$b';
              print $string,"\n";

              # perl4 prints: $b 2 $a $b
              # perl5 prints: 1 2 $a $b

     * "m//g" attaches its state to the searched string

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          "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)

              $_ = "ababab";
              while(m/ab/g){
                  &doit("blah");
              }
              sub doit{local($_) = shift; print "Got $_ "}

              # perl4 prints: Got blah Got blah Got blah Got blah
              # perl5 prints: infinite loop blah...

     * "m//o" used within an anonymous sub
          Currently, if you use the "m//o" qualifier on a regular
          expression within an anonymous sub, all closures gen-
          erated from that anonymous sub will use the regular
          expression as it was compiled when it was used the very
          first time in any such closure.  For instance, if you
          say

              sub build_match {
                  my($left,$right) = @_;
                  return sub { $_[0] =~ /$left stuff $right/o; };
              }
              $good = build_match('foo','bar');
              $bad = build_match('baz','blarch');
              print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
              print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
              print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";

          For most builds of Perl5, this will print: ok not ok
          not ok

          build_match() will always return a sub which matches
          the contents of $left and $right as they were the first
          time that build_match() was called, not as they are in
          the current call.

     * $+ isn't set to whole match
          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:

     * Substitution now returns null string if it fails
          substitution now returns the null string if it fails

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              $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.

     * "s`lhs`rhs`" is now a normal substitution
          "s`lhs`rhs`" (using backticks) is now a normal substi-
          tution, with no backtick expansion

              $string = "";
              $string =~ s`^`hostname`;
              print $string, "\n";

              # perl4 prints: <the local hostname>
              # perl5 prints: hostname

     * Stricter parsing of variables in regular expressions
          Stricter parsing of variables used in regular expres-
          sions

              s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;

              # 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';
              $_ = 'bar';
              s/^([^$grpc]*$grpc[$opt]?)/foo/;
              print;

              # perl4 prints: foo
              # perl5 prints: foobar

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

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              $test = "once";
              sub match { $test =~ m?once?; }
              &match();
              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

     * Failed matches don't reset the match variables
          Unlike in Ruby, failed matches in Perl do not reset the
          match variables ($1, $2, ..., $`, ...).

     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.

     * Barewords that used to look like strings look like subroutine
      calls
          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 now main'SeeYa
              # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")

          Use -w to catch this one

     * Reverse is no longer allowed as the name of a sort subroutine
          reverse is no longer allowed as the name of a sort sub-
          routine.

              sub reverse{ print "yup "; $a <=> $b }
              print sort reverse (2,1,3);

              # perl4 prints: yup yup 123
              # perl5 prints: 123
              # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()

     * "warn()" won't let you specify a filehandle.
          Although it _always_ printed to STDERR, warn() would

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          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 resets signal handler correctly
          Under HPUX, and some other SysV OSes, one had to reset
          any signal handler, within  the signal handler func-
          tion, each time a signal was handled with perl4.  With
          perl5, the reset is now done correctly.  Any code rely-
          ing on the handler _not_ being reset will have to be
          reworked.

          Since version 5.002, Perl uses sigaction() under SysV.

              sub gotit {
                  print "Got @_... ";
              }
              $SIG{'INT'} = 'gotit';

              $| = 1;
              $pid = fork;
              if ($pid) {
                  kill('INT', $pid);
                  sleep(1);
                  kill('INT', $pid);
              } else {
                  while (1) {sleep(10);}
              }

              # perl4 (HPUX) prints: Got INT...
              # perl5 (HPUX) prints: Got INT... Got INT...

     * SysV "seek()" appends correctly
          Under SysV OSes, "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.

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              open(TEST,">>seek.test");
              $start = tell TEST;
              foreach(1 .. 9){
                  print TEST "$_ ";
              }
              $end = tell TEST;
              seek(TEST,$start,0);
              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 inter-
     polated within certain expressions, statements, contexts, or
     whatever.

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

              print "To: someone@somewhere.com\n";

              # perl4 prints: To:someone@somewhere.com
              # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
              # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string

     * Double-
          quoted strings may no longer end with an unescaped $
          Double-quoted strings may no longer end with an unes-
          caped $.

              $foo = "foo$";
              print "foo is $foo\n";

              # perl4 prints: foo is foo$
              # perl5 errors: Final $ should be \$ or $name

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

     * Arbitrary expressions are evaluated inside braces within double
      quotes
          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}|";

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              # perl4 prints: |@{w.w.w}|foo|
              # perl5 prints: |buz|bar|

          Note that you can "use strict;" to ward off such trap-
          piness under perl5.

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

              $s = "a reference";
              $x = *s;
              print "this is $$x\n";

              # perl4 prints: this is XXXx   (XXX is the current pid)
              # perl5 prints: this is a reference

     * Creation of hashes on the fly with "eval "EXPR"" requires pro-
      tection
          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

          Changing

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

          to

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

          causes the following result:

              # perl4 prints: Nope
              # perl5 prints: Yup

          or, changing to

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

          causes the following result:

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

     * Bugs in earlier perl versions
          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

     * Array and hash brackets during interpolation
          You also have to be careful about array and hash brack-
          ets during interpolation.

              print "$foo["

              perl 4 prints: [
              perl 5 prints: syntax error

              print "$foo{"

              perl 4 prints: {
              perl 5 prints: syntax error

          Perl 5 is expecting to find an index or key name fol-
          lowing the respective brackets, as well as an ending
          bracket of the appropriate type.  In order to mimic the
          behavior of Perl 4, you must escape the bracket like
          so.

              print "$foo\[";
              print "$foo\{";

     * Interpolation of "\$$foo{bar}"
          Similarly, watch out for: "\$$foo{bar}"

              $foo = "baz";
              print "\$$foo{bar}\n";

              # perl4 prints: $baz{bar}
              # perl5 prints: $

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

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     * "qq()" string passed to "eval" will not find string terminator
          "qq()" string passed to "eval"

              eval qq(
                  foreach \$y (keys %\$x\) {
                      \$count++;
                  }
              );

              # perl4 runs this ok
              # perl5 prints: Can't find string terminator ")"

     DBM Traps

     General DBM traps.

     * Perl5 must have been linked with same dbm/ndbm as the default
      for "dbmopen()"
          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 exceeding limit on the key/value size will cause perl5 to
      exit immediately
          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 exceed-
          ing 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.
              YUP

              # perl5 prints:
              dbm store returned -1, errno 28, key "trap" at - line 3.

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     Unclassified Traps

     Everything else.

     * "require"/"do" trap using returned value
          If the file doit.pl has:

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

          And the do.pl file has the following single line:

              return 3;

          Running doit.pl gives the following:

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

          Same behavior if you replace "do" with "require".

     * "split" on empty string with LIMIT specified
              $string = '';
              @list = split(/foo/, $string, 2)

          Perl4 returns a one element list containing the empty
          string but Perl5 returns an empty list.

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

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