With CGI.pm, you ought to be able to do something like this untested code:

use CGI
$q = new CGI;
print $q->header(-status=>'401 Authorization required');
# add code here to print the rest of the error document
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__________
He seemed like such a nice guy to his neighbors / Kept to himself and never bothered them with favors
- Jefferson Airplane, "Assassin"