my %cache = ('-1' => qr/([^\(\)]+)\)+\Z/);

sub peel {
  my $str = shift;
  my $level = shift || 0;
  if (!exists $cache{$level}) {
    my $opening = '[^\(]+\(' x $level;
    my $closing = '\)' x $level;
    $cache{$level} = qr/\A$opening(.+?)$closing\Z/;
  }
  $str =~ $cache{$level};
  return $1;
}