Use of uninitialized value $class in concatenation (.) or string at Animal.pl line 7.
 Cow goes moooo!
Use of uninitialized value $class in concatenation (.) or string at Animal.pl line 13.
 Horse goes neigh!
Use of uninitialized value $class in concatenation (.) or string at Animal.pl line 19.
 Sheep goes baaaah!