So you want to hack on PIGGE, eh? Good for you! The following steps should get you started. Please let me know if you encounter any problems!
The first step of course is to get PIGGE working on your system. Without this, the rest of this process is bound to be a bit frustrating. Of course, if you're working on porting PIGGE, this may be exactly what you're trying to fix. In which case, thank you!
You'll want to start by working through the installation guide. Don't forget to run the tests! If you have any problems, either with functionality or performance, check the troubleshooting document.
The ports contain a wealth of information, showing how to configure a number of important settings, create scenes from various types of content files, script new commands, and so on. Understanding what's going on with the ports will help you understand why some things are the way they are in the code.
The documentation, especially the internals documents, contains a wealth of information that will help you attack the code. As time goes on, I will be adding to these, so be sure to check them again if you update to a new release of PIGGE.
You'll probably want to start by understanding PIGGE::Main and its parents, PIGGE::MainBase, PIGGE::Config, and PIGGE::Commands. For those of you familiar with OpenGL or interested in the underlying rendering code, check out PIGGE::Driver::SDLOpenGL. After that, I'd suggest bouncing around the source tree as your whim takes you; there is a module guide to help you find your way around.