$ vi p6.pl
"p6.pl" 36 lines, 925 characters 
#!/opt/perl5.16/bin/perl

use strict;
use warnings;

our $paren = qr/                # Need declared variable with use strict.
      \(
        (
           [^()]+               # Not parens
         |
           (??{ $paren })       # Another balanced group (not interpolated yet)
        )*
      \)
    /x;                         # 'x' means ignore whitespace, comments.


my $stuff = "On the outside now then (we go( in( and in (&stop)(awhile) ( furthe
r ))) but still (here) ) and now (for a while) we are out again.";
$stuff =~ /($paren)[^()]*($paren)/;

print "----------\n";
print "$stuff\n";
print "----------\n";
"p6.pl" 27 lines, 650 characters 
$ ./p6.pl 
Variable "$paren" is not imported at (re_eval 1) line 2.
Global symbol "$paren" requires explicit package name at (re_eval 1) line 2.
Compilation failed in regexp at ./p6.pl line 14.
$