use 5.010;
use Algorithm::Combinatorics qw(variations_with_repetition);
say join q(), @{ $_ } 
    for variations_with_repetition [qw(ABC EFG KLM XYZ)], 2;
__END__
ABCABC
ABCEFG
ABCKLM
ABCXYZ
EFGABC
EFGEFG
EFGKLM
EFGXYZ
KLMABC
KLMEFG
KLMKLM
KLMXYZ
XYZABC
XYZEFG
XYZKLM
XYZXYZ