my @letters = qw( A B C D E F );
my $len = 4;

sub convert {
   my ($num) = @_;

   use integer;

   my $base = @letters;
   my $rv = '';

   for (1..$len) {
      $rv = $letters[$num % $base] . $rv;
      $num /= $base;
   }

   return $rv;
}


print(convert($_), "\n")
   for 0..@letters**$len-1;