use WWW::Mechanize;
my $browser = WWW::Mechanize->new(
    cookie_jar => {},
    autocheck => 1
);
$browser->get(
    'http://users.guardian.co.uk/signin/0,12930,-1,00.html'
);
$browser->form_name('regpss1') || die "$!";
$browser->set_fields(
    AU_LOGIN_ID => 'my login',
    AU_PASSWORD => 'my password'
  );
$browser->submit() || die "$!";
print $browser->content();