$ cat p2.pl
#!/opt/perl5.16/bin/perl

use strict;
use warnings;

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

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

print "----------\n";
print "$stuff\n";
print "----------\n";
print $1 . "\n";
print "----------\n";