$ cat p7.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)[^()]*($paren)/;

print "-original-\n";
print "$stuff\n";
print "1---------\n";
print 'X' . $1 . 'X' . "\n";
print "2---------\n";
print 'X' . $2 . 'X' . "\n";
print "3---------\n";
print 'X' . $3 . 'X' . "\n";
print "4---------\n";
print 'X' . $4 . 'X' . "\n";
print "5---------\n";
print 'X' . $5 . 'X' . "\n";
print "6---------\n";
print 'X' . $6 . 'X' . "\n";
print "----------\n";
$ ./p7.pl 
-original-
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.
1---------
X(we go( in( and in (&stop)(awhile) ( further ))) but still (here) )X
2---------
X X
3---------
X((for a while))X
4---------
X(for a while)X
5---------
Use of uninitialized value $5 in concatenation (.) or string at ./p7.pl line 31.
XX
6---------
Use of uninitialized value $6 in concatenation (.) or string at ./p7.pl line 33.
XX
----------
$