#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

for my $string (qw( ab1b2b3d
                    ab1b2x
              )) {
    my @two;
    my @matches = $string =~ /(a)
                              (b.(?{ push @two, $2 if defined $2 }))+
                              (d)/x;
    say for @matches, '---', @two, '======';
}