Risking to echo footpad, I suggest you use Perl to solve a real-world problem that interests you. Let me share my experience:

I became aware of the concept Local Shared Object (aka Adobe Flash Cookie) while listening to a podcast. There were privacy issues involved that triggered my attention, and since there were (at the time) no appropriate modules on CPAN, I ended up writing one that I released on CPAN.

As adamk, I have learned a lot by releasing code publicly, including

• proper testing
• version management
• module dependency management
• platform differences (e.g. endianness)
• supporting multiple versions of Perl