78727 Affiliate Modification

I have an affilaite script called Little Salesmen by Wes Blaylock that is written in perl - I am allowed to modify this for own use. I want to be able to change this script to pay commissions on monthly subscriptions rather than as it currently is paying on a single purchase of a product. If it could track the payment details and allow me to manually cancel a commission if someone was to cancel a subscription payment. If it is too difficult to change - what would it cost to write an affiliate script from scratch that tracks subscription payments?