Modern Perl 4th Edition by chromatic is out now, and the price of the digital version is $0.00.
I must disclose that I was the editor for this edition. Actually, I’m bragging about it. I’m proud to be involved with this classic reference on the classic Swiss Army knife language. And “classic” in the case of this language doesn’t mean over the hill. As chromatic points out, modern Perl may well be the best tool for the job you have to get done. And at this price, what is there to keep you from finding out?
If you already know and appreciate Perl and Modern Perl, you might want to get the print edition. It isn’t free, but it’s worth the price, and chromatic isn’t going to make a lot on royalties from $0.00.