Shane Warden (otherwise known as chromatic throughout the world of Perl) has released Modern Perl as a free book available under a Creative Commons license. This is an essential book for people programming in Perl especially those who might not be familiar with improvements in Perl 5. All too often people approach Perl without understanding some of the newer improvements to the language.

Why did he release it under a free license? He answers the question here:

Why give away the text of this book? Ultimately, we hope that the book is useful. Countless programmers and dabblers will benefit from learning how to use Perl 5 well. We believe we'll reach more of them by encouraging everyone to share this book than by trying to maximize our profits.

It is more about audience size than it is about profit, Free books FTW. If you use Perl, download this book, buy it, or better yet, just donate a couple of dollars to chromatic. There is no more authoritative source for Perl, and we should all do everything to encourage authors who are smart enough to publish free books.