I am not a graphics expert, but I Googled "perl png". Image::PNG was at the top.

At the bottom of the documentation, the author gives a quick survey of all of the modules that he thought could possibly compete with his.

(For example, Image::PNG::Rewriter may have some aspects that are similar to what you are working on.)

Anyhow, I skimmed through the list and thought you might be interested, too.

GD, by Lincoln D. Stein, was #3 in the Google search results; GD.pm is a Perl interface to Thomas Boutell's gd graphics library. Wikipedia says that Thomas Boutell was the editor of RFC 2083, PNG (Portable Network Graphics) Specification, Version 1.0.