my $alphabet = "0123456789ABCDEF";
  my %successor = $alphabet =~ /(.)(?=(.))/g;
  my $zero = substr($alphabet, 0, 1);
  ...
  sub genIterator {
    my($len, $count) = @_;
    my $cur = '';
    my $lastdig = substr($alphabet, $count - 1, 1);
    return sub {
      if (length($cur) < $len) {
        $cur .= $zero;
      } else {
        $cur =~ s/([^$lastdig])$lastdig*\z/$successor{$1}/
            or $cur = undef;
      }
      $cur;
    }
  }