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: ''