#!/usr/bin/perl -w
use strict;

sub strip_paren {
   my $input = $_[0];
   my $re;

   $re = qr/
        (?:
         \(
            (
              (?:
                [^()]+
                |
                (??{$re})
              )+
            )
         \)
       )
       |
       \(\)
       /x;

   my @output = $input =~ /$re/g;

   return @output;
}

sub print_crud {
   print shift, "\n";
   foreach my $line (@_) {
      print "  $line\n";
   }
   print "\n";
}

my $string = "A (B C D)";
print_crud($string, strip_paren($string));

$string = "A (B C D) (E F G)";
print_crud($string, strip_paren($string));

$string = "A B (C (D E) (F G)) (H I)";
print_crud($string, strip_paren($string));