sub InProduct(){ my($a, $b) = @_; return $a->X*$b->X+$a->Y*$b->Y+$a->Z*$b->Z}

A side note: The  InProduct() function is defined with a prototype that specifies an empty argument list, but the function only makes sense if it's called with two arguments. You are obviously calling the function in a way that defeats prototype checking; don't bother with prototypes in such cases — or in most other cases. See also Far More than Everything You've Ever Wanted to Know about Prototypes in Perl -- by Tom Christiansen.