Here is how you can create a new element in a HTML::Form, like the one used in WWW::Mechanize.
Why you will want to do such a thing ? Sometimes there is a <from>'s which use fields created on the fly with javascript, to emulate this behavior you can do the same i.e.

my $form = $mech_obj->form_name('myform');
$form->push_input('text', {name => 'field', value => 'blah'});
[download]

that is it. HtH