A (blessed) reference can overload stringification or eq or == or numerification(?) so any of them can fail. I just use == since it is so much simpler than eq when it comes to references (it doesn't involve constructing a multi-part string, just interpretting a pointer as an integer).

overload's StrVal() documentation recommends Scalar::Util's refaddr(), which would then allow you to use == without worrying about overloading.