sub userid {
   my $self = shift;
   $self->session->param('userid') 
      and return $self->session->param('userid');

   my $username = $self->authen->username 
      or return;

   $self->session->param( 
      'userid' => ($self->_get_user_id($username) or confess())
   );
}

sub _get_user_id(
   my ($self,$username) = @_;
   # your way of accessing the user id
   # even if it's heavy, it's called once per session
}