I know about PERL_RL, my problem is that ::Perl creates visual blemishes in my prompt, and that installing ::Gnu is so complicated on a Mac that doesn't have developer tools. Although I'd gone ahead and had my client install Xcode COmmand Line Tools as prerequesite to installing perlbrew, simply to create a local Perl environment in which to install a readline module, it turned out that cpanm Term::ReadLine::Gnu would require homebrew in order to install readline-gnu, at which point I decided (with a little exasperation) to try installing ::Perl instead.

I'm still waiting to hear back about whether kcott's suggestion above (make'ing ::Gnu manually) worked for my client. Unfortunately my two Macs both have Xcode and homebrew so I can't test.

I hope I haven't muddied the waters further! :-)