"... matching pairs..."

By chance I found this and slightly adopted it:

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dump;
use feature qw(say);

my @s = split //, "AAAA";
my @m = map { $s[$_] . ' ' . $s[ $_ + 1 ] } 0 .. $#s - 1;

dd \@m;

say scalar @m;

__END__
[download]

I'm still wondering why it works ;-)

Update: Getting the letters:

dd grep { $_ ne "" } split /[^A]/, q(AAGAATGCCAGTTTGTAAGTACTGACTTTGGAAAA);

Regards, Karl