Math::IntMod => module to perform integer arithmetic module N. That could be done easyly on top of Math::BigInt or even Math::GMPn (for faster operation). The N constant could be specified when the number is created of using a lexical pragma (storing it inside %^H). See also emulate 32-bit on perl 64.
rdiff => Using the rsync protocol write a program/module able to generate the same output as diff but doing it between a local and a remote file or directory.
GitHub installer => An application similar to cpanm that can download and install perl modules from GitHub. It should follow dependencies and allow the user to decide what comes from github and what from CPAN.