Win8 Strawberry 5.8.9.5 (32)  Sun 09/27/2020 14:19:34
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

use Data::Dump qw(dd);

my $string = 'ACGTAAAAATGCCCATGGGGGGG';

my @repeats = do {
    my $p;
    grep { $p = !$p } $string =~ m{ ((.) \2+) }xmsg;
    };

dd \@repeats;

__END__
["AAAAA", "CCC", "GGGGGGG"]