The subroutine variant requires a reference to callback
       routine as the second argument to request() and it can
       also take an optional chuck size as the third argument.
       This variant can be used to construct "pipe-lined" pro-
       cessing, where processing of received chuncks can begin
       before the complete data has arrived.  The callback func-
       tion is called with 3 arguments: the data received this
       time, a reference to the response object and a reference
       to the protocol object.  The response object returned from
       request() will have empty content.  If the request fails,
       then the the callback routine is called, and the
       response->content might not be empty.

       The request can be aborted by calling die() in the call-
       back routine.  The die message will be available as the
       "X-Died" special response header field.

       The library also allows you to use a subroutine reference
       as content in the request object.  This subroutine should
       return the content (possibly in pieces) when called.  It
       should return an empty string when there is no more con-
       tent.