2) .... but you must clear the "CPANPLUS cache" when shifting from one installation to the other (or use some other alternatives listed in that URL)

No you don't. Unless you go out of your way to disable automatic index refreshing, refreshing indexes isn't needed, it happens automatically when necessary.

3) CPANPLUS documentation is meager and practically non-existent. I found the URL quoted above by accident, and its parent URL is broken.

Not true, this conclusion isn't supported by reality.

That page you found via googling is dated from 2002, 11 years ago. It is outdated and obsolete.

The cpanp/CPANPLUS/File::Fetch documentation is current , complete, comprehensive.

it shows (the same as cpanp help )  x                   # reload CPAN indices