I do lots of image work with Perl. You really need ImageMagick or some other image software. You don't want to be attempting to manage image formats yourself. ImageMagick is the best one I've found with Perl. There may be something simple you could do with gimp if you have access to it (it comes wtih most Linux installs as an option).

But the other reply was correct: Visible watermarks aren't much of a deterrent for theft since someone can easily get rid of them knowing they're there. And there's no tracking tied to them, etc.

Digimarc has the best invisible watermarking (fingerprinting) I've reviewed. I believe they have some sort of related facility for tracking, but I haven't gotten that far with their software. My understanding is that the Digimarc watermark has to be the last thing you apply for it to remain accurate. So if you apply another watermark, or even compress, after adding the Digimarc watermark it becomes useless. We also found that for some small, compressed images, there's not enough "room" for Digimarc to add their most robust watermark. If you have their software, they'll probably run those stats for you on a given image set.

ImageMagick is very good IMO. You might want to join their mailing list if you have specific questions. Their web site has details.