Which means that the source isn't getting filtered properly, and so the filtered source ends up with the same case statements that are in your original source--but case isn't a valid Perl bareword, so that's why you get that error.

But that's simply the symptom, really.

The *real* problem is this: Text::Balanced::_match_quotelike is being called, and it matches from the first '?' to the last (the one in the comment), and it sees that as a valid regex. In case this doesn't quite make sense, it sees this:

    ? 'foo' : 'bar';

    switch('x'){
      case 'y'              {print "y\n"}
      else                  {print "x\n"}
    }

    # ?
[download]

    if ($foo =~ ?bar?)
[download]

Here's a runthrough of what's going on, essentially; it makes the most sense if you follow along in the original source.

• Switch.pm receives your program source, which looks like this:

      my $x = 2 ? 'foo' : 'bar';
  
      switch('x'){
        case 'y'              {print "y\n"}
        else                  {print "x\n"}
      }
  
      # ?
  [download]

  It steps through that source, matching against regular expressions, in the form /\Gpattern/gc. This means that each time the regex engine tries to do a match, it starts in the source string *after* the last successful match. So using pos, I tried to figure out what it was doing.

• The first match is at the end of the lexing loop. It's supposed to match what looks like whitespace followed by non-whitespace, and it matches this:

  
      my
  [download]

  That's three characters: a newline, then 'my'. (I realize it's hard to see the newline. :) This puts pos at 3.
• The next thing it matches is done using Text::Balanced::_match_variable, and it matches:

       $x
  [download]

  Which is also three characters: a space, then '$x'. This puts pos at 6.
• After this we go through a few other match attempts, then are back down to the non-whitespace followed by whitespace, which matches:

       =
  [download]

  Which is two chars: a space, followed by '='. This puts pos at 8.
• Go through again, get back to non-whitespace followed by whitespace. We match:

       2
  [download]

  Which is two chars: a space, then '2'. This puts pos at 10.
• All of this matching is going on in a loop, and each time through the loop we're trying to match against a bunch of other things, as well. One rather important thing is each time we try to match switch/case statements. We haven't done so yet, because we've been doing other things. :)

  We've also been trying to match "things that look like quotes", using Text::Balanced::_match_quotelike. Nothing has matched yet, but the next time we go through, we do:

  In _match_quotelike we first match the '$pre' condition, which is optional whitespace. This moves pos to 11. The next thing in the string is '?': _match_quotelike looks at this and enters an if condition, looking to see if this could be a regex match. It looks to see if there's another '?' in the string--and there is! It's hidden in a comment, but _match_quotelike doesn't notice, and so the "quote" matched is:

      ? 'foo' : 'bar';
  
      switch('x'){
        case 'y'              {print "y\n"}
        else                  {print "x\n"}
      }
  
      # ?
  [download]

  This puts pos at 124. _match_quotelike returns this match, and this puts us at the end of the string: we're done.
• Since we're done, that means we didn't match any of the switch/case statements, which means that they got through unfiltered. They get passed directly to Perl, which doesn't like them, hence the error.

But it seems to fit pretty well.

UPDATE: I just tested this on Perl 5.6.0, and I got errors there, as well. So it looks like it's matching in 5.6.0 as a '?'-delimited regex as well. The error is slighty different:

syntax error at t.pl line 9, near "){"
String found where operator expected at t.pl line 10, near "case 'y'"
        (Do you need to predeclare case?)
Execution of t.pl aborted due to compilation errors.
[download]

i have no idea, but i did try this at the end:

=for comment
why does this break?
=cut
[download]

oh yeah - running version Perl 5.005_03 with version 1.83 of Text::Balanced

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=linux, osvers=2.2.5-22smp, archname=i386-linux
    uname='linux porky.devel.redhat.com 2.2.5-22smp #1 smp wed jun 2 0
+9:11:51 edt 1999 i686 unknown '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O2 -m486 -fno-strength-reduce', gccversion=egc
+s-2.91.66 19990314/Linux (egcs-1.1.2 release)
    cppflags='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    ccflags ='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    stdchar='char', d_stdstdio=undef, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -ldl -lm -lc -lposix -lcrypt
    libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynami
+c'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'


Characteristics of this binary (from libperl): 
  Built under linux
  Compiled at Feb  2 2000 15:35:58
  @INC:
    /usr/lib/perl5/5.00503/i386-linux
    /usr/lib/perl5/5.00503
    /usr/lib/perl5/site_perl/5.005/i386-linux
    /usr/lib/perl5/site_perl/5.005
    .
[download]

R-R-R--R-R-R--R-R-R--R-R-R--R-R-R--
L-L--L-L--L-L--L-L--L-L--L-L--L-L--

The hardest part about parsing Perl is that you have to track both entries in the symbol table and token types in the input stream in order to decide when / is division vs. a regular expression and when ? is the trinary operator vs. a regular expression.

I seriously doubt that Switch.pm will ever solve this problem completely for Perl5 simply because the effort involved is more than a sane person would do.

Anyway, I wanted to strongly discourage the use of such complicated magic for any serious (a.k.a. production) scripts. You could easily find no problem with it at first and then make a minor change and find yourself either making "voodoo" changes (like remove '?'s from comments) in order to make Switch.pm happy or having to do a major rewrite to remove your dependence on that module.

I'll see if I can dig up an example of this different error message. In the meantime:

% perl petruchio
syntax error at petruchio line 8, near "){"
String found where operator expected at petruchio line 9, near "case 'y'"
        (Do you need to predeclare case?)
Execution of petruchio aborted due to compilation errors.
% perl -MText::Balanced -le 'print $Text::Balanced::VERSION'
1.83

% perl -V
Summary of my perl5 (revision 5.0 version 6 subversion 0) configuration:
  Platform:
    osname=solaris, osvers=2.6, archname=sun4-solaris
    uname='sunos ouessant 5.6 generic_105181-05 sun4u sparc sunw,ultra-4 '
    config_args=''
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef
    useperlio=undef d_sfio=undef uselargefiles=define
    use64bitint=undef use64bitall=undef uselongdouble=undef usesocks=undef
  Compiler:
    cc='gcc', optimize='-O', gccversion=2.8.1
    cppflags='-I/usr/local/include'
    ccflags ='-I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    stdchar='unsigned char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, usemymalloc=y, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =' -L/usr/local/lib '
    libpth=/usr/local/lib /lib /usr/lib /usr/ccs/lib
    libs=-lsocket -lnsl -ldl -lm -lc -lcrypt -lsec
    libc=/lib/libc.so, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
    cccdlflags='-fPIC', lddlflags='-G -L/usr/local/lib'


Characteristics of this binary (from libperl):
  Compile-time options: USE_LARGE_FILES
  Built under solaris
  Compiled at Sep 26 2000 18:27:15
  [...]

Summary of my perl5 (revision 5.0 version 6 subversion 0) configuratio
+n:
  Platform:
    osname=linux, osvers=2.2.17-compact, archname=i686-linux-thread-mu
+lti
    uname='linux sickle 2.2.17-compact #1 sun jun 25 01:10:11 mst 2000
+ i686 unknown '
    config_args='-des -Dcc=gcc -Dcf_email=ActivePerl@ActiveState.com -
+Uinstallusrbinperl -Dusethreads -Duseithreads -Dprefix=/usr/local/Act
+ivePerl-5.6'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=define use5005threads=undef useithreads=define usemulti
+plicity=define
    useperlio=undef d_sfio=undef uselargefiles=define 
    use64bitint=undef use64bitall=undef uselongdouble=undef usesocks=u
+ndef
  Compiler:
    cc='gcc', optimize='-O2', gccversion=2.95.2 20000220 (Debian GNU/L
+inux)
    cppflags='-D_REENTRANT -fno-strict-aliasing -I/usr/local/include'
    ccflags ='-D_REENTRANT -fno-strict-aliasing -I/usr/local/include -
+D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    stdchar='char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
+ lseeksize=8
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -ldl -lm -lpthread -lc -lposix -lcrypt
    libc=/lib/libc-2.1.3.so, so=so, useshrplib=false, libperl=libperl.
+a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynami
+c'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'


Characteristics of this binary (from libperl): 
  Compile-time options: MULTIPLICITY USE_ITHREADS USE_LARGE_FILES PERL
+_IMPLICIT_CONTEXT
  Locally applied patches:
      ActivePerl Build 618
  Built under linux
  Compiled at Sep 12 2000 17:00:49
  @INC:
    /usr/local/ActivePerl-5.6/lib/5.6.0/i686-linux-thread-multi
    /usr/local/ActivePerl-5.6/lib/5.6.0
    /usr/local/ActivePerl-5.6/lib/site_perl/5.6.0/i686-linux-thread-mu
+lti
    /usr/local/ActivePerl-5.6/lib/site_perl/5.6.0
    /usr/local/ActivePerl-5.6/lib/site_perl
[download]

    String found where operator expected at switch.pl line 9, near "ca
+se 'y'"
        (Do you need to predeclare case?)
    syntax error at switch.pl line 8, near "){"
    Execution of switch.pl aborted due to compilation errors.
[download]

    [Clip]: Cleep!! Cleep cleep cleep cleep!
    [John]: What is it Clip? What is it girl?
    [Clip]: Cleep! Cleep cleep cleep. Cleep cleep!
    [John]: What! Uninitialised variable stuck under a tractor?
    [Clip]: Cleep!
[download]

#! /usr/local/bin/perl -w

use 5.006;
use strict;
use warnings;

use Switch;

my $exit = 0;

sub shell
{
   print $/; # Error goes away if you remove this line, or change to "
+\n";

   while(1)
   {
      switch ("cmd")
      {
         case /^quit/   { $exit = 1; }
         case /^help/   { help(); }
      }
      last if $exit;
   }
}
[download]

I guess we'll just have to wait for Perl6 (RFC here).
--
Brovnik