To close this thread:   there's “a wrong way” and “a right way” to do many things in this life. “Both of them work,” but one of them works much better.

Such is the case here. For o conf makepl_arg, “the wrong way” is to use PREFIX=, as I had done, and “the right way” is to use SITEPREFIX= instead.

“I over-simplify, of course,” but if you do have the luxury of choice, this is a better arrangement.

To make this problem “really go away for good,” I basically wiped-out the old site-perl directory and rebuilt it after making this configuration-change. The resulting directory structure was vastly simpler and more sane, and the problems that started this thread went away and never came back.