One wonders, erm, why? The order is neither specified nor significant (apart from a general recommended ordering of header types), so why do you think it's of any use?

(Having said that, $r->headers_in returns an APR::Table instance rather than a true hash, and that class supports a method $tbl->do( $callback ) which iterates over the items in the table passing key/value paris to the callback (presumably in order as the APR::Table docs specifies keys are stored in the order added). That might get what you're looking for.)