At risk of upsetting likbez:

use strict;
use warnings;

my $string = "AAAATTTAGTTCTTAAGGCTGACATCACGTCAGCGTTACCCCCCAAGATTGGGGAC
+TTT";
my $len = 0;
my $best = '';

$best = $1, $len = length $1 while $string =~ /((.)\2{$len,})/g;
print "best: $best ($len)\n"
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Prints:

best: CCCCCC (6)
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