Perl 5.16, not 5.6.
The sum of 1,...,n = n(n+1)/2, not n(n-1)/2. I realized I'd made a
mistake here while reading Gwen a book about hippos who arrive for a
party sequentially in groups of 1,...,n who then leave in reverse order.
The book ends with the original hippo missing the other 44, but you get
only 36 hippos total with n(n-1)/2, not 45 like you should.