First Perl job, I was reusing (old) Perl code, and having zero Perl experience I just took the code for granted and made modifications to it. The author didn't use any modules and the code was quite lengthy. Later I found out about FTP and date handling modules. I was able to reduce the code by 80% or so and the code was also more robust (and no longer platform dependent!). This was a real eye-opener for me. Nowadays I start out by looking on CPAN instead of reinventing wheels:)