I personally believe

While most people would say that Perl 5 and Perl 4 (as opposed to Perl 6 wrt Perl 5) are two versions of the same language, what you propose would certainly be sensible. I think that there does not exist an extremely strict policy on "this kind of things." Just to be fussy I would note that whatever the title says, the main text does mention Perl 5, but not to the point of amending the vastly incorrect information it reports. Having a separate Perl 5 entry, would still require to correct the current Perl 4 ("and previous") one anyway. Again, the approach seems sensible, since e.g. both diagrams at "diagram & history of programming languages" are precisely following it. Incidentally, the diagram "extracted" from HOPL is much more impressive than any other one I have seen thus far, and gives an idea of how rich the project is, and of its accuracy, thus it strikes as even more unfortunate that Perl is not treated adequately! Fortunately, wiki-like mechanism or not, there's a reference email address available.

Thus I propose that:

1. a first mail is sent to the maintainer to ask whether two separate entries for Perl (meaning "Perl 4" or less) and Perl 5 respectively would be preferred over a single one or vice versa;
2. we concoct up here, collectively, a good text for the new entries (or entry) in accordance with HOPL's style.

I believe that the latter task could be accomplished by means of followups to the root node of this very thread, of which I must say, however, that unfortunately doesn't seem to have gained enough momentum to be the best "place" to do so; so, if you know of a better one, I'm all open to suggestions. In the meanwhile, since it was me to bring all this up, I may send the first mail myself anytime soon now.