#!/usr/bin/env perl

use 5.014;
use warnings;

my $string = 'AAATTTAGTTCTTAAGGCTGACATCGGTTTACGTCAGCGTTACCCCCCAAGTTATTGGGGACTTT';
my $min_repeat = 2;

for my $base (qw{A C G T}) {
    say "$base: ", get_longest_length($string, $base, $min_repeat);
}

sub get_longest_length {
    my ($str, $base, $min) = @_;

    my $re = '[' . 'ACGT' =~ s/$base//r . ']+';
    return (
        sort { length $b <=> length $a }
        grep length $_ >= $min, split /$re/, $str
    )[0];
}

##</code><code>##

A: AAA
C: CCCCCC
G: GGGG
T: TTT