It's probably UNIVERSAL::can.

Update: Yep, looks like it. UNIVERSAL is Perl's implicit base class for all objects. can queries whether the object has that method (including via inheritance). It looks like the Net::DNS docs are suggesting to use duck typing here to tell whether the Net::DNS::RR subclass has the address method. An alternative would be to use isa (also from UNIVERSAL) to see if an object is of a certain class (or is one of its subclasses) and therefore supports that class's methods. The thing about that is though that you'd have to test against a whole list of Net::DNS::RR subclasses. As long as all of the subclasses' address methods behave the same, duck typing is safe here, and easier.