# Read in
my @my_a = <PLAIN>;
my @my_b = <CTR>;

# XOR
my @my_c = map { $my_a[$_] ^ $my_b[$_] } 0..$#my_a;
my @my_d = map { $my_b[$_] ^ $my_c[$_] } 0..$#my_b;

# Check
foreach (0..$#my_d)
{
  $my_d[$_] eq $my_a[$_]
    or die "d[$my_d[$_]] != a[$my_a[$_]] at line $_\n";
}

print "OK\n";