Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
Perl4 and perl5 are informal names for different versions of the Perl programming language. It's easier to say ``perl5'' than it is to say ``the
5(.004) release of Perl'', but some people have interpreted this to mean there's a language called ``perl5'', which isn't the case. Perl5 is merely the popular name for the fifth major release (October 1994), while perl4 was the fourth major release (March 1991). There was also a perl1 (in January 1988), a perl2 (June 1988), and a perl3 (October 1989).
The 5.0 release is, essentially, a complete rewrite of the perl source code from the ground up. It has been modularized, object-oriented, tweaked, trimmed, and optimized until it almost doesn't look like the old code. However, the interface is mostly the same, and compatibility with previous releases is very high.
To avoid the ``what language is perl5?'' confusion, some people prefer to simply use ``perl'' to refer to the latest version of perl and avoid using ``perl5'' altogether. It's not really that big a deal, though.
See the perlhist manpage for a history of Perl revisions.