# CGI::Application code...

# The main runmode, where I show the page
#
sub foo
{
    my $self = shift;
    my $s    = $self->session();

    my $tmpl = $self->load_tmpl('foo');

    # ... load data in the template, as long
    # as the missing and invalid fields, that
    # come from the session (see foo_update()).

    foo_view( $tmpl, $data, $s->param('missing'), $s->param('invalid'));

    return $tmpl->output();
}

sub foo_update
{
    my $self = shift;
    my $q    = $self->query();
    my $s    = $self->session();

    my %form = $q->Vars();                                                      
    my $results = Data::FormValidator->check( \%form, $profile );               
                                                                                
    if ($results->has_invalid() or $results->has_missing()) {                   
                                                                                
        $s->param( missing => [ $results->missing() ] ,                         
                   invalid => [ $results->invalid() ] );                        
                                                                                
        $self->redirect( { rm => 'foo' } );                                     
    }                                                                           
    else {                                                                      
                                                                                
        # Data are good, we proceed...                                          
        $self->redirect( { rm => 'bar' } );                                     
    }                                                                           
}                                                                               

##</code><code>##

# This is called by foo() from the main application, just before
# to display the page
#
sub foo_view
{
    my $template   = shift;
    my $date       = shift;
    my $missing    = shift;
    my $invalid    = shift;

    $template->param( "missing_$_" => 1 ) for @$missing;
    $template->param( "invalid_$_" => 1 ) for @$invalid;

    # [...]

    return $template;
}

##</code><code>##

<TMPL_IF invalid_some_field>Bad boy! This is an error message.</TMPL_IF>