It was authored by Larry Wall, is available free, and is maintained by him with help and suggestions from thousands of users. It was not meant as a comprehensive guide to the language, but it includes most of the core things you need to understand and use it. The first chapter introduces the language by starting with a simple program. After explaining this first bit of code, the author adds more code and further explanation. This goes on through more than a dozen additions, all the while adding more constructs and concepts to build a complete, fairly complex program by chapter's end.
Every step is one simple concept, clearly explained and illustrated.
Without fail, the examples make understanding easy and could be a model for other teaching books. If you work with UNIX, you need Perl and this book. Maybe the clue for other potential authors is that the author, Schwartz, says he wrote most of the book on his portable computer at McMenamin's Pub in Beaverton. The Camel Book Programming Perl is a more comprehensive guide to Perl. It presumes that you are familiar with UNIX and have some substantial programming experience.