Do some stuff. What doesn't matter -- just solve some real problem you have, using Perl. As you do it, look for opportunities to use modules that solve a piece of your problem (manipulating files, downloading web content, parsing data, whatever). And then, for each module that you use, read it. Read the code. Figure out what the weirdness you see is all about. The whole great thing about CPAN is not just that you can use the libraries; it's that they're in source and you can read (and hell, modify) them.