use Regexp::Common;

    my $balanced = qr/[^()]+|$RE{balanced}{-parens=>'()'}/;

    sub extract {
        my ($want, $from) = @_;
        my $nested = qr/$balanced* \Q$want\E $balanced*/x;
        $from =~ m/( \( $nested \) | \Q$want\E )/x;
        return $1;
    }

    my $expr = '(((B,G),A,(A,B)),C,(D,(E))),F';

    for ('A'..'H') {
        print "$_: ", extract($_,$expr),"\n";
    }