If you want to do a lot of work with images and don't want to use external modules, then can I suggest you find a targa to pbm (portable bit map) converter so that you can work with the images in pbm format (or ppm if you want color as well). That is about the simplest image format available, and is very easy to manipulate with straight perl.

I am pretty sure that the netpbm package comes with a pbmtotga and tgatopbm program to do the conversion for you.

Here is a description of the ppm image format: http://netpbm.sourceforge.net/doc/ppm.html