A linear solution, which will work fast for any size string: 
use strict;
use warnings;

my ($c, $maxn, $n, $maxc, $str) = ('', 0);
$str = join '', <DATA>;

for (0..(length($str)-1)) {
    $_ = substr($str, $_, 1);
    if ($_ ne $c) {
        $n = 1;
        $c = $_;
    }
    else {
        $n++;
        if ($n > $maxn) {
            $maxn = $n;
            $maxc = $c;
        }
    }
}

print $maxc x $maxn;

__DATA__
ABDBACCBBBCBDDBCDCBCCDBABCBABBBBBADACDABACC

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

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