Mod_perl side:
use MD5;

$date = get_todays_shortdate();  # 11/05/2004
$Private_secret = "This is my private server password"
$data = "this is my signed data."

$digest = MD5->hash("$date:$Private_secret:$data");
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Send the digest and the data over to the other server and it knows the Private_secret and can verify that the data has been signed by constructing the same string calling md5->hash on it and comparing the two digests... If the hacker does not know the private_secret or the layout of the digest string then they cant forge the $data sig.