sub navbar_callback {
    my ($self, $ht_params, $tmpl_params, $tmpl_file) = @_;

    if( $tmpl_file eq 'master.tmpl' )
    {
        # load your variables
        my %navbar_params = $self->navbar_params;
        $tmpl_params->{$_} = $navbar_params{$_} foreach keys %navbar_params;

        # alternatively, you could also load a separate template for the navbar, and
        # pass it on to the master as a tmpl_var. Note that the if() clause protects you
        # from an infinite loop
        # $tmpl_params->{navbar} = $self->navbar_template;
    }
}