$ perldoc CGI |grep -i xhtml -C1

    -no_xhtml
        By default, CGI.pm versions 2.69 and higher emit XHTML
        (http://www.w3.org/TR/xhtml1/). The -no_xhtml pragma disables 
+this
        feature. Thanks to Michalis Kabrianis <kabrianis@hellug.gr> fo
+r this
--
        If start_html()'s -dtd parameter specifies an HTML 2.0, 3.2, 4
+.0 or
        4.01 DTD, XHTML will automatically be disabled without needing
+ to
        use this pragma.
--
    The -encoding argument can be used to specify the character set fo
+r
    XHTML. It defaults to iso-8859-1 if not specified.

--
    Otherwise it will be replaced by the default DTD. If the public DT
+D
    contains 'XHTML', CGI.pm will emit XML.

    The -declare_xml argument, when used in conjunction with XHTML, wi
+ll put
    a <?xml> declaration at the top of the HTML header. The sole purpo
+se of
--
        action: this script
        enctype: application/x-www-form-urlencoded for non-XHTML
                 multipart/form-data for XHTML, see multipart/form-dat
+a below.

--

        If XHTML is activated (the default), then forms will be
        automatically created using this type of encoding.
--
    The start_form() method uses the older form of encoding by default
    unless XHTML is requested. If you want to use the newer form of en
+coding
    by default, you can call start_multipart_form() instead of start_f
+orm().
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