Perl Versions
In this book, I refer to the latest "stable" version of Perl, which is 5.8.3 as of this writing. The vast majority of what I say works unaltered on older versions of Perl 5, but not Perl 4. If you use any version of Perl older than 5.004_04, you should upgrade for reasons unconnected with features: 5.003 had issues such as security problems and memory leaks. You can find out the version number of your perl by passing it the -v flag:
% perl -v
This is perl, v5.8.3 built for i586-linux
Copyright 1987-2003, Larry Wall
[...]
Perl won't execute a script named on the command line if the -v flag is present. A more detailed description of your perl's configuration can be obtained with the -V flag; if you use the perlbug program that comes with perl to issue a bug report, it automatically includes this information in your report.
A separate development track exists for Perl; you will know if you have one of those versions because the release number either contains an underscore followed by a number of 50 or larger or contains an odd number between two dots. Nothing is guaranteed to work in such a distribution; it's intended for testing. If you find you have one and you didn't want it, the person who downloaded your perl probably visited the wrong FTP link. This happens more often than most people would think; take a moment to check your perl if you're not sure.
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