Anyone have a good argument for preferring -Mstrict -Mv5.10 against -Mv5.12 ??

Personally I'd use -wMstrict or -wM5.012. v5.12 was released more than 9 years ago (vs. more than 11 years for v5.10), so I think it's ok to require it. (Then again, if I'm only requiring v5.10 for the // operator, I might just leave the version requirement off entirely.)

Test::More and Capture::Tiny

That's more or less how I've done it in the past, sometimes using IPC::Run3 and more recently my own module IPC::Run3::Shell. Here is one example, along with most of the other test scripts in that directory.