Through unit testing.

While some other languages get you in bad habits of writing massive monolithic chunks of code before you try to (compile and) run it, Perl is ideally suited to running frequent tests and writing tests scripts to hit the code. My eyes will gloss over things that actually running it won't. It's not a great use of time for me to read my own code again - but writing a re-usable test to verify it works, and it still works after things change, that's where it's at.

Alternate answer: on my coworker's screen. While I'm not likely to see bugs or a better approach in my own code, someone else sure is. It's enormously instructive to have an equally experienced or better programmer review your code, and one of the best ways to learn new techniques.