So,what if I can only use array to implement, and Is there a method or package that can squash the memory occupied by "undef" in an array and return the the positions back when I try to fill something into the position that was occupied by a undef, sort of like autovivification?

Why won't a hash do if you don't want to spend the memory? Why do you think someone will have written a module that basically does what a hash does?