Actually, Perl is correct. $obj->dump() would "work" in your case. There is no difference between a subroutine in a package and a method of an object blessed into said package, at least as far as Perl is concerned. The arrow-call syntax $obj->method(@args) is little more than syntactic sugar around TestPackage::method($obj, @args).

There is no real concept of an interface in Perl other than the documentation of methods of a class/package.