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:

# Header.pm
sub set_header {
    $header_ref->{ $key } = $value;
}
[download]

"(be) liberal with your input" does the following:

# plugins/foo
set_header( $main::header_ref, '-status' => '304 Not Modified' );
[download]

Your comment meant a lot to me.