I don't know anything about the perl cron scripts as I prefer to use the PHP scripts.
But as per the php script:
<b>Fatal error</b>: Call to undefined function: send_mail() in <b>/path/to/2pop/ubbthreads/cron/php/subscriptions.php</b> on line <b>173</b><br />
The 6.4 cron script won't work on 6.3 as the mail function was complete rewritten - so the script looks for send_mail which doesn't exist yet in 6.3.
