Oh Fabulous Bangers! Thank you very much!

Exactly what I was looking for - and I can now see how you managed to stitch together a mild case of recusion... and the embeded map/map cartesian product - rather nifty indeed!

I have made one slight performance tweak:

my $count = scalar @$expanded;
$expanded = [ 
  map { 
    my $row = $_;
    map { 
      my $newrow = {%$row};
      @$newrow{keys %$_} = values %$_ unless $newrow->{$parent_key};
      $newrow }  @$expansion ;
  } @$expanded ];

push @$expanded,  @$expansion if $count == scalar @$expanded;
[download]

to:

if ( $expanded->[0]{$parent_key} ) {
  push @$expanded,  @$children;
}
else {
  # create cartesian product against existing $expanded
  $expanded = [
    map {
      my $row = $_;
      map {
        my $newrow = {%$row};             # clone expanded row
        @$newrow{keys %$_} = values %$_;  # add in parent_key details
        $newrow                           # return new row
      }  @$children;
    } @$expanded ];
}
[download]

as this tests for the presence of $parent_key once, rather than using the before/after count to decide if all the children are new.

Mucho kudos! and many many thanks

Jeff