Regexes. Before I was introduced to Perl (4 briefly, but 5 soon after), I had been taking classes about automata, language parsing, etc. The regex capability was killer.

Before that, I'd mostly been using Pascal (because of work), and doing any kind of matching the hard way was a PITA.

However, one of my classes had a token lab joint project, to practice "network" communications. My contribution came from the Camel book, almost verbatim. Another win.