No, I didn't see that thread. It didn't occur to me to search for stuff regarding Expect.
IO::React looks really cool. I glanced at the code, and it's only a few hundred lines long, which is crazy, because it's an Expect replacement, and it's tiny compared to it. The only potential problem of using it is that it hasn't been updated in 11 years! But its tests still pass, but then you look at its tests, and its just one file called basic.t that only has like 18 tests in it, and basic.t manually prints 1..18 and ok and not ok implementing its TAP all on its own! That might not have been that crazy back in 2001.
Using TCL would be a really crazy dependency, but could work.
I think I'm leaning towards IO::React or Mocking it using default values and AutoReply = 1 like Term::UI does.