sub radix_sort {
    my ($data, $k) = @_;
    $k = !!$k;  # turn any true value into 1
    
    if ($k) { $k < length and $k = length for @$data }
    else { $k = length $data->[0] }
    
    while ($k--) {
      my @buckets;
      for (@$data) {
        my $c = substr $_, $k, 1;  # get char
        $c = "\0" if not defined $c;
        push @{ $buckets[ord($c)] }, $_;
      }
    
      @$data = map @$_, @buckets;  # expand array refs
    }
  }