If you "use warnings;" in the package, you get these helpful messages:

Use of uninitialized value $class in concatenation (.) or string at An
+imal.pl line 7.
 Cow goes moooo!
Use of uninitialized value $class in concatenation (.) or string at An
+imal.pl line 13.
 Horse goes neigh!
Use of uninitialized value $class in concatenation (.) or string at An
+imal.pl line 19.
 Sheep goes baaaah!
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for the stuff you say is "Non working code".

This gives a clear hint on what $class is set to when invoked that way, and why it "does not work".