This question concerns Perl’s class syntax (new as of Perl 5.38.0). I have two classes, Rectangle and Square, with Square inheriting from Rectangle. They have different constructor arguments; Rectangle expects length & height while Square expects just side. My hope was that Square would be able to inject the length and height arguments into the constructor for Square‘s parent Rectangle object, based on side‘s value. Something akin to this, though the BUILD syntax from Object::Pad doesn’t work: