I find it troubling that you don't have any specification at all. I would have presumed that, if you were doing this for an employer or business partner, that they would have provided a specification on exactly how the cards are to be generated, etc.

My advice to you, if your aim is indeed legitimate, is not to use perl to generate these numbers yourself, but instead seek the services of a professional gift card company. There are quite a few of them out there. Google for terms such as "stored value cards". This is money we're talking about. Don't screw around with it by trying to do an amateur hack job. There are too many caveats with stored value systems for a beginner to implement a system like this without putting their money and their reputation at risk.