I suggest you start with my book recommendations. But if those don't seem to meet your needs then feel free to browse the links I provide on over 165 available Perl books.

Top 100 Free Books Free Chapters

Beginning Perl Advanced Perl

Beginning CGI Advanced CGI

Free Books (downloadable)
The truth is that almost any beginner's book can give you a reasonable start at learning a new language. Some are definitely better than others, but if you cannot afford to purchase your book of choice, then by all means go with whichever free one is available! Your progress usually depends far more on the quality of the effort you put into studying than it does on the quality of the book.

I definitely prefer to study from paper books, but downloadable books (eBooks) have the advantage of supporting an electronic search. Here are several free books you'll want to consider downloading.

Online Only

• CGI Programming on the World Wide Web
  
• Perl 5 Internals
  
• Picking Up Perl
  
• Web Client Programming with Perl
  

Downloadable

• Beginning Perl
  
• Extreme Perl
  
• Impatient Perl
  
• Learning Perl the Hard Way
  
• Objected Oriented Perl
  
• Perl 5 by Example
  
• Perl for the Web
  
• Picking Up Perl
  
• Practical mod_perl
  
• The Perl Reference Guide
  

Libraries of Free Books
These sites have multiple books available, not just the Perl books I've listed above. So you might want to check out the sites to see if they have other titles of interest to you.

• O'reilly Open Books
  
• Google Open Books Project
  
• Perl Programming