It is "Programming Perl", by Larry Wall and Randal Schwartz. It changed my developer life for two main reasons.
The first reason is that I read the ugly and horrible italian translation, and I found so many errors, typos and code mistakes that learning Perl became a real adventure with such a book. Therefore, it changed my life because it was the book that made me switch to English only tech books. They are the most up-to-date and do not contain silly translation errors.
The second reason is that it showed me how beauty and nice programming can be. It was not tied to Perl itself, but the way the authors were expressing the language. They were showing to the world how they loved programming, how fun can be to automate tasks and to administer complex systems.
Today I appreciate only books by authors that can show me how beauty a technology is, and not those authors that try to simply explain how to use it. I can read the manpages for that!