You can pass the form data as an array ref of pairs, as you've done, then the http headers as the same in a third argument:

$req = POST 'http://gino/ciccio.cgi',
            [
              sessionid => 'something',
              mittente => 'something',
              prefix => 'number',
              numtel => 'number',
              messagetext => 'message',
              flash => "0",
              Submit => " Invia "
            ],
            [
              'Header1' => "header1thing",
              'Header2' => "header2thing"
            ];
[download]

$req is a perfectly good HTTP::Request object.

After Compline,
Zaxo