#!/usr/bin/perl -w
use strict;
use XML::Twig;

my $xml = XML::Twig->new(
              keep_spaces_in => [ 'pre' ],    # safer, other tags might need to be included
              pretty_print => 'indented',
              twig_roots => { 
                  'body' => \&insert_form_tags, 
              },
          );
$xml->parsefile('index.html');

$xml->print;

sub insert_form_tags() {
    my ($t, $body) = @_;
    my $form= $body->insert(
        form => {
            method => "Post",
            action => "submit.cgi"
            },
        );
    $form->insert_new_elt( first_child => input => { type => "text", name => "foo_is" });
}