use Digest::MD5 qw(md5_hex);
my $ip=$r->connection->remote_ip();
my $url="/gotohere/";
my $basekey="somespecialkeyequalinsiteaandsiteb";
my $sid=md5_hex("$ip;;$url;;$basekey");
$url="http://www.siteb.com$url?sid=$sid";
$r->redirect($url);