Three part longish question to explore the scope of Perl's power.
in reply to What's the biggest piece of work you've done alone with Perl?
If you're exploring Perl's power, I suggest you rethink your first question, or consider adding a fourth. Perl is suitable for more than just solo programming. Teams can use it, too. I once managed a team that had written an 80KLOC application server in Perl. (The core of the team that wrote it was ~6 people, more were added later to extend it).
Personally, the biggest single thing I have in Perl is about 1500 lines, plus another 1000 lines of unit tests, but not including CPAN stuff (HTML::Template, DBI, plus a couple of others).
A strategy that helped me pull it off was to use Test-Driven Development. Figuring out how to write tests first helped me think through APIs with an actual client (the test) in mind. That helped avoid a lot of nonsense. Having a large body of unit tests gave me confidence that I wasn't breaking things as I went.
The biggest mistake I made was not getting the code body under source-code control soon enough. (I've since set up a home CVS pserver.)