#!/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, '======';
}
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

That doesn't seem to yield any output that I can understand. Why three 'b1' 'b2' 'b3' from ab1b2b3b4d and nothing from any of the others?

c:\@Work\Perl\monks>perl -e
"use warnings;
 use strict;
 use feature qw{ say };
 ;;
 for my $string (qw(ab1b2b3b4d  ab5b6b7d  ab8b9d  abxd  ad)) {
   my @two;
   my @matches = $string =~ /(a)
                             (b.(?{ push @two, $2 if defined $2 }))+
                             (d)/x;
   say for @matches, '---', @two, '======';
   }
"
a
b4
d
---
b1
b2
b3
======
a
b7
d
---
======
a
b9
d
---
======
a
bx
d
---
======
---
======
[download]

Update: "This is perl 5, version 14, subversion 4 (v5.14.4) built for MSWin32-x86-multi-thread"

---

Give a man a fish:  <%-{-{-{-<

Weird, I'm getting a different output:

a
b4
d
---
b1
b2
b3
======
a
b7
d
---
b5
b6
======
a
b9
d
---
b8
======
a
bx
d
---
======
---
======
[download]

This is perl 5, version 20, subversion 1 (v5.20.1) built for x86_64-linux-thread-multi

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

This one works for me.

#!/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";
  }
[download]