Definedness: Explain the difference between false and undefined. Show how to test for each.

Hashes: Show how to test for the existence of a hash key.

Autovivification: Show how to fetch the value in $foo->{'bar'}->{'baz'} without causing $foo->{'bar'} to spring into existence if it didn't exist to begin with.

Sorting: Show how to sort "foo123", "bar123", "baz345", "baz123" numerically first, alphabetically second.

List manipulation: Show how to find how many times the number 5 exists in map {int(rand(6)+1)} 1..20.