Your call to $douban->Users never calls the AUTOLOAD sub, but the accessor method of the same name in the super class, which returns undef.

BTW this: if($name !~ /^Users|Subjects|Reviews$/){ probably doesn't do what you want, you'd need /^(?:Users|Subjects|Review)$/ (because the alternation | has looser precedence since the concatenation which is used for the anchors ^ and $).