What You Should Already Know
To get the most out of this book, you should already know the basics of Perl (specifically, Perl version 5). You should be familiar enough with Perl to be able to at least read some code and know what the code is doing. You don't have to be a Perl guru or Perl hacker to learn Perl/Tk, but it will help if you feel comfortable with the language. Here's the laundry list of things you should at least recognize: hashes, arrays, subroutines, and their anonymous versions, $_ and @_.
Perl/Tk utilizes the object-oriented features available in Perl 5, so even if you don't completely understand them, you should be able to recognize them when you see them. The only other thing you'll need is your prior knowledge of other graphical user interfaces (GUIs) and what you did and did not like about them. This helps when deciding what features to include in your own applications. Take a look at the word processor you use on your PC, your web browser, or any program that has buttons and scrollbars and accepts both mouse and keyboard input.
Those applications are pretty major ones; we'll start with much simpler examples and build up from there. We'll be covering each basic widget and all its associated options in detail. You'll learn how to make a window look the way you want it to look. You'll also learn how to make a window user-friendly and attractive.
If you want to know more about Perl in general, you should read Learning Perl, Programming Perl, Advanced Perl Programming ...