my $paren_rx;
$paren_rx = qr{
  \(
  (?:
    (??{ $paren_rx })         # either match another paren-set
    | (?s: [^\\()]+ | \\. )+  # or match non-parens (or escaped parens)
  )?
  \)
}x;

my ($chunk) = $string =~ /((??{$paren_rx}))/;