By having a problem to solve.
I had a huge set of scripts written in awk but eventually I needed more than it had to offer. I dabbled with Java, but at that time there wasn't a standard regexp library, and those that were available were poor.
I needed a more expressive language and Perl seemed the obvious choice being a C/Linux/sed/awk/shell kinda guy.
Although I bought Learning Perl and Programming Perl, the book that helped me most was the awesome Perl Cookbook, but I guess that's because I already knew how to program.