That error is coming from HTML::Template in this line:

$template->param( Main_navigation_menu        => \@DBRows );
[download]

... not the session. I suspect a dump of @DBRows would illuminate the problem.

Update: After a second look at this, if you were to send a link to someone with rm=Page this'll probably may go boom because there's been nothing placed in that session parameter (since the user never went to "Startup" first. Perhaps some logic in the BuildPage runmode that will force them over to the Startup Page if the session var isn't set. CGI::Application::Plugin::Forward is handy for this.