Have a look at Modern Perl by chromatic, pages 40–47. This book is available free online at http://modernperlbooks.com/books/modern_perl/.

Update: See also

• Perl variable types
• Data: Hashes (Associative Arrays)
• Beginning Perl by Simon Cozens, Chapter 3, “Lists and Hashes,” available free online at http://learn.perl.org/books/beginning-perl/
• Impatient Perl by Greg London, Section 2.3, “Hashes,” available free online at http://www.greglondon.com/iperl/index.htm
• QandASection: hashes
• Data Type: Hash.

HTH,