This book, written by Perlmonk ([...]) David Cross, is an excellent, easy to read, and easy to follow guide into what Perl does best: Data Munging. For those who don't know, Munging Data means taking data from one format and putting it into another. Perl excels at this, and the author shows you the how and the why.
The author gives you enough information, and background to start working with the more advanced Perl functions like map, grep, pack, unpack, etc. It is possible to write Perl without ever having to use these modules, but David Cross shows you how they are more effective, more powerful. This book will expand your Perl vocabulary by leaps and bounds.
I know that some people would say that the book is too thin, and it is thinner than many computer books today, but the thickness of a book does not determine it's merit. Effective Perl Programming by Joseph Hall and Randal Schwartz is often cited as one of the best Perl books ever and it's thinner than this one.
If you are a junior to intermediate level programmer, and you want to improve your Perl skills, pick up this book. You won't be disappointed.