#! 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;
}