Also, did you spot the (intentional) bug?
It's in the C<_validate_birth_date()> method. We should check that
-that value in C<$birth_date> is actually defined and object before we
-go and call C<isa()> on it! Leaving out those checks means our data
+the value in C<$birth_date> is actually defined and an object before
+we go and call C<isa()> on it! Leaving out those checks means our data
validation code could actually cause our program to die. Oops.
Note that if we add a superclass to Person we'll have to change the