In the context of running Perl 5.004 on DOS, all that is fine, of course, after all, there is more than one way to do it. I've written modules that I try to make as backwards-compatible as possible (although only down to 5.6, so far). I even wrote some QBASIC way back. So if it helps you, you're of course free to code that way (setting aside for a moment the usual arguments about future maintainability and so on). I might just suggest that your fake warnings.pm at least contain $^W=1;, and to be aware of the issues of two-arg open like I mentioned.

On the other hand, your post was a reply to a wisdom seeker who said they were just getting started (and not necessarily on 5.004 for DOS :-) ). Many of the suggestions you made go against modern best practices (useful variable names, don't use $a and $b outside of sort, three-arg open with lexical filehandles, using warnings, and using a recent version of Perl), and those best practices usually exist for a good reason. So I think it's important to be aware of that, and I'd suggest putting in a disclaimer.

Other than that, getting Perl to run in as many places as possible is certainly not a bad thing :-)