my $string = "ATTTAGTTCTTAAGGCTGACATCGGTTTACGTCAGCGTTACCCCCCAAGTTTTTTTTTTTTTTTTTTATTGGGGACTTT";

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

print "best: $best\n"