If the error message really comes from the line you've shown, then $self->{ObjA_ref} is an instance of MyClass and not of ClassA. So your error lies somewhere else, or you haven't shown us the real code. Data::Dumper on $self can be helpful here.

It helps us to help you better if you show self-contained code that exhibits the problem and is directly runnable by us, so we can easily replicate your problem.