you'd simply go through each function, disabling its explicit tests then checking your coverage.

FGS! If you comment out the tests of foo() in your test suite, then that is exactly the same as if you had no test suite.

And, as I showed above, if you have a function call in your code that doesn't have an associated function definition, then Devel::Cover doesn't detect it.

All you are doing it testing your tests, not your code.