As ccn quoted

There is a single iterator for each hash, shared by all each, keys, and values function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating keys HASH or values HASH.

So if at all unsure just do

  keys %{$patterns{$gen}})
[download]

before the while loop.

But, why does this not happen every time the subroutine is called -- just once in a while?

Is it possible that you sometimes return from the subroutine from inside the while loop?