delete $hash{ELEM} deletes the key named ELEM from the hash %hash. After this command, exists $hash{ELEM} will no longer be true. See exists for more information on the difference between the operators delete and undef when applied to a hash element (you usually want the former).
In fact, delete will accept a hash slice. So delete @hash{'a','b','c'} will delete the three keys 'a', 'b', 'c' from %hash.
The return value of this operator is the list of the values which were associated with the specified keys (prior to their deletion, of course). This return value is rarely used.