# In this example, 'first' column
are neighbours with only 1 position differ.
The rest of the columns are 2 positions differ

Seed = 000
100 110 120 130 101 102 103
200 210 220 230 201 202 203
300 310 320 330 301 302 303
010 011 012  013
020 021 022  023
030 031 032  033
001  
002  
003

Seed = 001
101 111 121 131 100 102 103   
201 211 221 231 200 202 203      
301 311 321 331 300 302 303      
011 010 012 013
021 020 022 023
031 030 032 033               
000
003
002     

Hence given a tag of length L
we will have 3*L + 9L(L-1)/2   neighbors
[download]

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
use Carp;

# If possible we prefer not to use any CPAN module
# or regex. It's for my understanding


my $seed = $ARGV[0] || "000"; 
# Seed can be longer than 3 digits but always 0,1,2,3

for (my $i = 0; $i < length($seed); $i++) {

    # First loop is to generate 1 position differ
    for (my $bs=1; $bs<=3; $bs++) {

        my $bval = $bs;
        if ( substr($seed,$i,1) == $bs) {
            $bval = 0;

        }

        my $ns = $seed;
        substr($ns,$i,1,$bval);
        print "$ns\n";


        # Second loop for tags in 2 positions differ

        for (my $j=($i+1); $j < length($seed); $j++) {

            for (my $cs = 1; $cs<=3;$cs++) {

                my $cval = $cs;
                if (substr($ns,$j,1) == $cs) {
                    $cval = $cs;
                }

                 my $ns2 = $ns;
                 substr($ns2,$j,1,$cval);
                 print "\t$ns2\n";

            }
        }


    }
[download]