You might want to have a look at the CPAN modules that fall under the Imager namespace. Imager.pm is the base class, and from there look at Imager::Filters. This module allows you to manipulate images in many ways, including adding "watermarks." Watermarks may be of programmer-definable strength, and can be put at any location in the image.

Also see, Imager::Font. Together with Imager::Filters, you could probably put a text graphic anywhere you want within another graphic image.

I hope this helps.