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"
[download]

Prints:

best: CCCCCC (6)
[download]

We can use (*SKIP) to avoid that:

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

print "best: $best\n"
[download]

But that is still not completely efficient: the regexp is recompiled at every loop iteration because of $len, so maybe the following simpler code could be faster:

my $best = "";
while ($string =~ /((.)\2+)/g) {
    $best = $1 if length $1 > length $best
}

print "best: $best\n"
[download]

Or maybe this more convoluted variation:

my $best = "";
$best = $1 while $string =~ /((.)\2*)(*SKIP)(?(?{length $^N <= length 
+$best})(*FAIL))/g;

print "best: $best\n"
[download]

Does that work?

Win8 Strawberry 5.30.3.1 (64)  Tue 09/29/2020 13:32:10
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

my $string = 'AABBBBCCC';

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

print "best: '$best' \n"

^Z
best: ''
[download]

---

Give a man a fish:  <%-{-{-{-<

Oops, no, it doesn't, but I think it should!

Is that a bug in perl?