use Algorithm::Bucketizer;
while (my $in = <>) {
    chomp($in);
    my %counts;
    $counts{$_}++ for split //, $in;
    my $b = Algorithm::Bucketizer->new(bucketsize=>length($in)/4);
    $b->add_item($_, $counts{$_}) for keys %counts;
    $b->optimize(maxrounds=>100);
    print $_->serial, " => ",
        join(", ", map {"$_ ($counts{$_})"} $_->items ),
        "\n" for $b->buckets;
}