use 5.18.0;
use warnings;

my $string = "AAATTTAGTTCTTAAGGCTGACATCGGTTTACGTCAGCGTTACCCCCCAAGTTATTGGGGACTTT";
my %subs;

$subs{$_}++ for grep { length >= 2 } split m/,/ => ($string =~ s/([ACGT])\K(?!\1)/,/gr);
printf "%5d %3d %s\n", $subs{$_->[1]}, @$_ for sort { $b->[0] <=> $a->[0] || $a->[1] cmp $b->[1] } map {[ length, $_ ]} keys %subs;

##</code><code>##

    1   6 CCCCCC
    1   4 GGGG
    1   3 AAA
    3   3 TTT
    2   2 AA
    2   2 GG
    5   2 TT