NAME
       perltrap - Perl traps for the unwary

DESCRIPTION
       The biggest trap of all is forgetting to use the -w
       switch; see the perlrun manpage.  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:

       o   The English module, loaded via

               use English;

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

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

       o   Curly brackets are required on ifs and whiles.

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

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

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

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

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

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

       o   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 the perlvar
           manpage.

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

       o   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.

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

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

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

       o   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.)

       o   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.)

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

       o   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    $;


       o   You cannot set $RS to a pattern, only a string.

       o   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:

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

       o   You must use elsif rather than else if.

       o   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.

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

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

       o   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.

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

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

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

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

       o   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:

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

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

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

       Shell Traps

       Sharp shell programmers should take note of the following:

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

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

       o   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.

       o   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).

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

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

       Perl Traps

       Practicing Perl Programmers should take note of the
       following:

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

       o   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.

       o   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 the perlop manpage.

       o   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.

       o   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!


       o   Remember not to use "=" when you need "=~"; these two
           constructs are quite different:

               $x =  /foo/;
               $x =~ /foo/;


       o   The do {} construct isn't a real loop that you can use
           loop control on.

       o   Use my() for local variables whenever you can get away
           with it (but see the perlform manpage 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.

       o   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 Traps

       Penitent Perl 4 Programmers should take note of the
       following incompatible changes that occurred between
       release 4 and release 5:

       o   @ now always interpolates an array in double-quotish
           strings.  Some programs may now need to use backslash
           to protect any @ that shouldn't interpolate.

       o   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.  For
           example:

               sub SeeYa { die "Hasta la vista, baby!" }
               $SIG{'QUIT'} = SeeYa;

           In Perl 4, that set the signal handler; in Perl 5, it
           actually calls the function!  You may use the -w
           switch to find such places.

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

       o   Double-colon is now a valid package separator in an
           identifier.  Thus these behave differently in perl4
           vs. perl5:
               print "$a::$b::$c\n";
               print "$var::abc::xyz\n";


       o   s'$lhs'$rhs' now does no interpolation on either side.
           It used to interpolate $lhs but not $rhs.

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

       o   These are now semantic errors because of precedence:

               shift @list + 20;
               $n = keys %map + 20;

           Because if that were to work, then this couldn't:

               sleep $dormancy + 20;


       o   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.

       o   open FOO || die is now incorrect.  You need parens
           around the filehandle.  While temporarily supported,
           using such a construct will generate a non-fatal (but
           non-suppressible) warning.

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

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

       o   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.

       o   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.

       o   m//g now attaches its state to the searched string
           rather than the regular expression.

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

       o   taintperl is no longer a separate executable.  There
           is now a -T switch to turn on tainting when it isn't
           turned on automatically.

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

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

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

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

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

       o   Setting $#array lower now discards array elements.

       o   delete() is not guaranteed to return the old value for
           tie()d arrays, since this capability may be onerous
           for some modules to implement.

       o   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.

       o   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.  To retain
           Perl 4 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 /x/, @list) { ... }

           to

               foreach $var (my @tmp = grep /x/, @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 $_.)

       o   Some error messages will be different.

       o   Some bugs may have been inadvertently removed.