use strict;
use warnings;

my $string = "AAAAAAATTTAGTTCTTAAGGCTGACATCGGTTTACGTCAGCGTTACCCCCCAAGTTTTTTTTTTTTTTTTTTATTGGGGACTTT";

my $len = 0;
my $best = "";
while ($string =~ /((.)\2{$len,})/g) {
    $len = length $1;
    $best = $1
}

print "best: $best\n"