Multiple Inheritance

        @ISA = ('Person');

        package Employee;
        @ISA = ('Person', 'Security::Agent', 'Programmer');

If $e is an Employee, then $e->languages_known looks first for Employee::languages_known
Then for Person::languages_known
Then in the superclasses of Person
Then in Security::Agent and its superclasses, then in Programmer and its superclasses
Then it gives up
Note Depth-First Search

        use base  ('Person', 'Security::Agent', 'Programmer');