LWP and cookies

 I am writing an application with LWP::UserAgent. From the web I can
authenticate on a form and then post some data on a second form.
From perl, the authentication is working fine, but as soon as I get
next form url, I receive from the server a "session timeout" message.
In my program I am using memory-cookies:

  my $ua = LWP::UserAgent->new();     # Create user agent object
    $ua->cookie_jar({});                # Cookie handling object
    $ua->agent('Mozilla/5.0');          # User agent property