In situations where you have deep trees of classes, there is a common situation in which you test a module 4 or 5 subclasses down, which should follow the correct behaviour of not just the subclass, but of all the parent classes.
This should be done to ensure that the implementation of a subclass has not somehow "broken" the object's behaviour in a more general sense.
Test::Object is a testing package designed to allow you to easily test what you believe is a valid object against the expected behaviour of all of the classes in its inheritance tree in one single call.
To do this, you "register" tests (in the form of CODE or function references) with Test::Object, with each test associated with a particular class.
When you call object_ok in your test script, Test::Object will check the object against all registered tests. For each class that your object responds to $object->isa($class) for, the appropriate testing function will be called.
Doing it this way allows adapter objects and other things that respond to isa differently that the default to still be tested against the classes that it is advertising itself as correctly.
This also means that more than one test might be "counted" for each call to object_ok. You should account for this correctly in your expected test count.