Presumably your code
int length = 5;
int hi[length];
is at local scope, not file scope, as the latter wouldn't be legal.
Are they still going in the data-segment
They were never going in the data-segment; they go on the stack (in typical/common implementations; the language standard doesn't say where they go).
why would I still have to call delete[] on the 2nd example, but not
the 1st example?
First, VLA's (variable length arrays like hi[length]) aren't legal in C++, so you couldn't call delete[]. But there's no need to call delete because hi goes out of scope at the end of the block it's in. An object or array allocated by new, OTOH, does not go out of scope until deleted. Only the pointer, hi, goes out of scope, but you may have assigned its value to another pointer that is still in scope.
