If $SubroutName is a string, &$SubroutName is a symbolic reference - after all, you are using a string as if it were a reference. And using a symbolic reference is what strict 'refs' disallows.

Either use no strict 'refs' around the call, use test(), or call it as a class method: main->$SubroutName is allowed, even with maximal strictness. But be aware that calling it as a class method implicitely passes the class as the first argument.