You try to receive the session ID via $cgi->param('sessionid'), but you never provide a session ID parameter in the URL or in a POST request.

Try this:

my $self_url = $cgi->self_url()
print qq{<a href="$self_url?session_id=$session_id">next page</a>\n}
[download]

And then use "next page" link. (Later on you should move the session ID to a cookie, but for texting the URL is fine).

Also please remeber to call ->flush() on your session object to actually store it to disk.