The CGI.pm online docs mention:

 $query->default_dtd('-//W3C//DTD HTML 4.01 Transitional//EN');
 print $query->start_html(
            -title  => 'Secrets of the Pyramids',
            -author => 'fred@capricorn.org',
            -dtd    => 1);
[download]

HTH,
Charles K. Clarkson