you replace 'foo' with an expression it can be hard to tell whether $f is a filehandle or a variable.

Nope, it is never a "filehandle" , there is no ambiguity

Here is a simple example that the compiler can't correctly handle without braces:

Sure it can, and it does (v5.14).

all you need for disambiguation is M as in m//

I don't know what exact justifications TheDamian gives in the book, but I highly doubt its because of that contrived example -- highly flimsy justification for such an obnoxious rule