I seem to remember a module that gets the script's directory, for just such a purpose.

You're probably thinking of FindBin - which is broken.

Perl doesn't complain about nonexisting directories in @INC, no.

You could check for the existence of your test modules with something like -f "./$foo" or -f "t/$foo" and bail if you can't find either.

Makeshifts last the longest.