#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11137405
use warnings;

my $random = join '', map qw(A C G T)[rand 4], 1 .. 50;

for my $seq ( qw( AGCAGC AATGCAATCGCAGCAGCA AGCTACCCAGCTAGGGAGCTA
  AAA_x_AAA_x_BBB_x_AAA_x_AAA_x_BBB
  ), $random )
  {
  my %found;
  $seq =~ /([A-Z]{3,}) .* \1 (?{ $found{$1}++ }) (*FAIL)/x;
  my %counts = map { $_, scalar(() = $seq =~ /$_/g) } keys %found;
  use Data::Dump 'dd'; dd "for sequence $seq", \%counts;
  }