#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11122267
use warnings;

my $string = "AAATTTAGTTCTTAAGGCTGACATCGGTTTACGTCAGCGTTACCCCCCAAGTTATTGGGGACTTT";
my @substrings;

push @{ $substrings[length $1] }, $1 while $string =~ /(([ACTG])\2+)/g;

my @sorted = map @{ $_ // [] }, reverse @substrings;

use Data::Dump 'dd'; dd \@sorted;