specific situation of wanting a user to own the apache processes it is fine, IMHO

It might be fine from an admin point of view. However, from what haj has said, it doesn't seem to solve the OP's problem.

I just did the experiment for the relevant portion (the output of top and ps, and indeed, the login name is not preserved

The OP wants the username to appear in top so they can differentiate between their admin account and a user running the Perl script. At least, I think that's what they want to do.

I recall a long time ago being introduced to nobody that was a *nix user account for running Perl scripts on a webserver. I didn't understand it at the time I don't remember enough details to try and piece it together now but it might jog the memory of some experts in *nix is it is a possible solution.