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.

In reply to Re^2: On creating hash whose keys aren't case-sensitive by anazawa
in thread On creating hash whose keys aren't case-sensitive by anazawa

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