Maybe look in your environment?

env | grep PERL
[download]

There is in fact a PERL5OPT environment variable that can load modules into every perl program by default, but most of us would consider that a bad habit to get into. it's really just meant for diagnostics and workarounds like PERL5OPT=-MCarp::Always perl myscript.pl for temporary increased debugging.

I think there might also be some place in /etc that can cause perl to load modules on startup. I don't use it though.

"use" is a good thing. It helps you document where your code is coming from, so that other people know what is going on. .... such as your own case :-) If the script you were using as a reference declared "use" lines for everything, you wouldn't need to ask how the mystery libraries got loaded.