sub next_iter {
    my($count, $die, $current) = @_;
    return '1' x $count unless $current;
    $current =~ s{
      (?!$die) ((.) $die*) $
    }{
      my $d = $2 + 1;
      $d x length $2
    }ex or return undef;
    return $current;
  }
  ...
  my $cur;
  print $cur while $cur = next_iter(3, 6, $cur);