Perl, like almost all popular software, has gone through much growth and change over the course of its 15-year life. The authors, Larry Wall and a large group of cohorts, publish new versions periodically. These new versions have been carefully designed to support most programs written under old versions, but occasionally some major new features are added that simply won't work with older versions of Perl.
This book assumes you have Perl Version 5 or higher installed. It's likely that if you have Perl already installed on your computer, it's Perl 5. But it's best to check. On a Unix or Linux system, or from an MS-DOS or Mac OS X command window, the perl -v command just shown displays the version number, in my case Version 5.8.0. The number 5.8.0 is "bigger" than 5, so I'm okay. If you get a smaller number (very likely 4.036), you'll have to install a recent version of Perl to enable the majority of programs in this book to run as shown.
What about future versions? Perl is always evolving, and Perl Version 6 is on the horizon. Will the code in this book still work in Perl 6? The answer is yes. Although Perl 6 is going to add some new things to the language, it should have no trouble with the Perl 5 code in this book.