use Some_RSA_Module;
use Digest::MD5 qw/ md5 /;

# decode base64 as before, then:

my $encrypted = substr($cookie, -64);

substr($cookie, -65) = '';      # strip the null and the security field

my $checksum = Some_RSA_Module::unencrypt($some_public_key, $encrypted);

my $new_checksum = md5($cookie);

if ($new_checksum eq $checksum) {
  print "Checksums match, cookie data is valid.\n";
} else {
  print "Checksums don't match, cookie data has been tampered with.\n";

}