Thanks to all three respondents. What happened was that @INC in my 5.10.1 didn't include all of the directories from past versions, such that (as I found out later) I needed an update of Term::ReadLine::Gnu to make one of my scripts work right. I thought the problems was with Term::ReadLine, and didn't check its status. Naturally, when I just said install, perl -MCPAN said it was up to date, so I tried to force it.

Perl -MCPAN's response of trying to unpack perl was mysterious at the end of a long day. Later I finally got @INC in 5.10.1 right, so that all past module directories are now present and in the right order.