#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $s = 'ABDBACCBBBCBDDBCDCBCCDBABCBABBBBBADACDABAC';
my $p = 0;
my $max = 0;

while ($s =~ /(?<=(.))(?!\1)/g) {
    my $l = pos($s) - $p;
    $max = $l if $l > $max;
    $p = pos $s;
}
say $max;