#!/usr/bin/perl
use strict;
use warnings; # "use warnings" tends to be preferred over "-w"

# Seed can be longer than 3 digits but always 0,1,2,3
my $seed = $ARGV[0] || "000";
if ($seed =~ /[^0-3]/) {  # "If $seed contains anything other than 0,1,2,3"
  die "Invalid seed.  Seed may only contain digits from 0 through 3.\n";
}

# First loop is to generate 1 position differ
for my $i (0..length($seed) - 1) {
  for my $first_diff (0..3) {
    next if (substr($seed, $i, 1) == $first_diff);

    my $outer_variant = $seed;
    substr($outer_variant, $i, 1) = $first_diff;
    print "$outer_variant";

    # Second loop for tags in 2 positions differ
    for my $j ($i+1..length($seed) - 1) {
      for my $second_diff (0..3) {
        next if (substr($outer_variant, $j, 1) == $second_diff);

        my $inner_variant = $outer_variant;
        substr($inner_variant, $j, 1) = $second_diff;
        print "\t$inner_variant";
      }
    }
    print "\n";
  }
}