my @letters = qw( A B C D E F );

for (0..6**4-1) {
   use integer;
   my $num = join '', map $letters[$_], $_/6/6/6 % 6,
                                        $_/6/6   % 6,
                                        $_/6     % 6,
                                        $_       % 6;
   print("$num\n");
}