#!/usr/bin/perl
use warnings;
use strict;

for my $string (qw(ab1b2b3b4d  ab5b6b7d  ab8b9d  abxd  ad)) {
  my @two;
  my @matches = $string =~ /(a)
                             (?:(b.)(?{ push @two, $2 }))+
                             (d)/x;

  @matches and splice @matches, 1, 1, @two;
  print "@matches\n";
  }