sub last_closed {
    my $last = $+[0];
    my $len = length $^N;
    for (1 .. $#+) {
      return $_ if defined($+[$_])
        && $+[$_] == $last
        && $+[$_] == $len + $-[$_];
    }
    return undef;
  }

  "ad" =~ m{
    (
       (a) (?{ print last_closed() })
       (b)? (?{ print last_closed() })
       (c?) (?{ print last_closed() })
       d
    ) (?{ print last_closed() })
  }x;