I've recently gotten into WWW::Mechanize... i would try this instead...

#!/usr/bin/perl
use strict;
use warnings;
use WWW::Mechanize;

my $user= 'XXXX@spamgourmet.com';
my $password= 'XXXX';

#maybe need cookies?
my $n=WWW::Mechanize->new(cookie_jar => {});

$n->get( q(http://www.nintendo-europe.com/NOE/de/DE/logout/index.jsp?D
+PSLogout=true) );
$n->form_name('login');
$n->set_visible( $user, $password );

#specify form to submit
$n->submit_form(form_name => 'login');

print $n->content()
[download]

just by quickly looking at it... set_visible might be your best bet since those fields don't have specific names. And as far as I can tell... those are the first two (non-hidden) fields in that form.

If it still doesn't work, try debugging by taking a look at $n->forms(). That should return an object of all of the form elements that mech sees and give you a better idea what to use for the fields. hope this helps.