If you've made it through Learning Perl and Intermediate Perl, you've probably been waiting with no small impatience for a book like this to round off the trilogy and your basic Perl education. If you're interested in Perl as a complete programming language, or want to be able to read and understand the rest of the Perl literature, then you need this.
Note that the thrust of the book is about providing the information you need to use Perl to build applications, so there's nothing about Perl internals, or embedding Perl or dropping down to C to speed things up. For that sort of thing, you might want to look at the various editions of Advanced Perl Programming.
Stylistically, Mastering Perl is a bit of a departure from the previous two books in the series. Gone is the tutorial feel, and there's no overarching pop culture theme to the examples. Instead, you're assumed to be competent and ready to develop your own code, and brian d foy's style treats the reader as an equal.
There are two types of material covered in the book. The first rounds off the rest of the Perl language not covered in the first two books. These are all things which are not exactly necessary for every day programming, but which anyone motivated sufficiently to learn enough Perl to be interested in this book will just want to know. Typeglobs, the symbol table and tied variables top this list. Additionally, there are excellent chapters on error handling and advanced regular expressions, the latter of which introduces the options and anchors used in lexing, and look-ahead and look-behind assertions.
The other material covers useful libraries for developing in Perl. Examples include chapters devoted to documentation with POD, serialization, logging, debugging, profiling, and benchmarking. These are all comprehensive and use fairly long examples with non-trivial code.
You could cobble together a minority of the material presented in this book from other sources (e.g. some of the stuff on ties or the symbol table), such as Effective Perl Programming, Perl Medic, Perl Debugged or Object Oriented Perl (and there's a helpful Appendix which recommends several such books as further reading), but having it presented here in one cohesive whole is a far superior learning experience, thanks to the author's clear explanations and copious examples. Additionally, there's stuff here that you just won't find in those other books (e.g. do you know what the PROPAGATE method does on an object?). I'd like to think I've read most of the important Perl books, but I still learnt a lot, and it filled in a lot of holes. It's bang up to date, too, which many of the other books you'd otherwise be relying on can't claim to be.
Overall, this fills a gaping void in the Perl literature and provides a suitable bridge between Intermediate Perl and the likes of Perl Cookbook and Perl Best Practices. It's an excellent, focussed book which provides almost everything you need to do real Perl programming.