TIMTOWTDI. You can also get the positions of the "borders", i.e. places where the character sequences change: 
#!/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;

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Longest possible run of a single character by choroba
in thread Longest possible run of a single character by srdst13

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