Do things you have never done before. Here's some suggestions.

• Instead of using modules from CPAN write your own
• Write an indexer
• Write functions/methods that manipulate 2d arrays
• Write functions/methods that manipulate 3d arrays
• Learn bitwise and bitshifting operators
• Write a module which interfaces with a shared object
• Learn curses

Personally I'd recommend skipping writing GUIs in perl.