note anazawa Thanks for your suggestion. Although I didn't understand why your code worked so, I confirmed your sub worked in an expected way. I didn't consider whether to normalize users' inputs. In my case, "be strict with your output" points out the following: <c> # Header.pm sub set_header { $header_ref->{ $key } = $value; } </c> "(be) liberal with your input" does the following: <c> # plugins/foo set_header( $main::header_ref, '-status' => '304 Not Modified' ); </c> Your comment meant a lot to me. 954718 954748