my $string = 'AAATTTAGTTCTTAAGGCTGACATCGGTTTACGTCAGCGTTACCCCCCAAGTTATTGGGGACTTT';

my %max;
$string =~ s[(.)\1*][if ( length($&) > ($max{$1} // 0) ) {$max{$1} = length($&); }]eg;

for my $k (sort keys %max) {
    print"$max{$k} ", $k x $max{$k}, "\n";
}

##</code><code>##

3 AAA
6 CCCCCC
4 GGGG
3 TTT