Is there any chance that set_callback() hangs on to the first callbacks you pass for 'list' and 'status', and ignores subsequent attempts to set them? If so, those callbacks will reference variables in the stack frame of the first invocation of foldertree() and not the variables in the stack frames of recursive invocations. That is, you'd get the right number of references in the array, but the values in the referenced hashes would be bad.

You could verify this by adding a print to the 'status' callback, and then dump %folderhash right before you recurse. If the numbers don't match, then you'll need to make the callbacks reference package variables instead.