Or you could simply use a foreach loop like so:

use warnings;
use strict;

my @three_mers = qw( ABC EFG KLM XYZ);

foreach my $mers_first (@three_mers) {
    foreach my $mers_sec (@three_mers) {
        print $mers_first, $mers_sec, $/;
    }
}
[download]

ABCABC
ABCEFG
ABCKLM
ABCXYZ
EFGABC
EFGEFG
EFGKLM
EFGXYZ
KLMABC
KLMEFG
KLMKLM
KLMXYZ
XYZABC
XYZEFG
XYZKLM
XYZXYZ
[download]