#! perl
use strict;
use warnings;
use List::MoreUtils 'uniq';

my $string = '0a0a0a0a0b0b0a0a0c0c0c0c0c0c0c0c' .
             '1f1f2b2b2b2b3e3e7b7b7b7b7b7b7b7b' .
             '8f8f8f8f8f8f8f8f6c6c4b4b4b4b3f3f' .
             '9d9d0f0f0f0f0f0f0f0f3a3a2e2e2e2e';

my @seqs = $string =~ /(([0-9a-fA-F]{2})\2)/g;
   @seqs = uniq grep { length == 4 } @seqs;

for my $seq (@seqs)
{
    my $matches = () = $string =~ /$seq/g;
    printf "%s: %d\n", $seq, $matches;
}

##</code><code>##

17:30 >perl 914_SoPW.pl
0a0a: 3
0b0b: 1
0c0c: 4
1f1f: 1
2b2b: 2
3e3e: 1
7b7b: 4
8f8f: 4
6c6c: 1
4b4b: 2
3f3f: 1
9d9d: 1
0f0f: 4
3a3a: 1
2e2e: 2

17:30 >

##</code><code>##

0a0axxx0a0a0yyyy
^^^^   ^^^^