Yes, something going on at line 1617 of CGI.pm at github, inside sub "header" -- 
if (($MOD_PERL >= 1) && !$nph) {
        $self->r->send_cgi_header($header);
        return '';
}

[download]
which explains the empty set being returned, after it's invoked a "send_cgi_header" mod_perl method that seems to be designed to take the place of sub "header" as a practical matter.

In reply to Re^3: mod_perl and CGI behavior by davebaker
in thread mod_perl and CGI behavior by naChoZ

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