No, I don't mean "useless use of foo in void context". I mean something rather more specific, along the lines of "comma operator encountered in scalar context; throwing away whatever precedes it".

Bear in mind, sometimes the thing that's thrown away isn't a constant. It can be (the return value of) a function call, for instance, and where is the "useless use of foo" warning then?

I realize there are probably rare situations in which an advanced programmer uses the scalar comma deliberately, but that's why recent versions of perl have added the ability to pragmatically set aside specific types of warnings within a lexical block. In the overwhelming majority of cases, the scalar comma is introduced by programmer error, and the warning would be beneficial.