if (data_submitted()) {
    @errors = validate_data();
    if (@errors) {
        output_errors(@errors);
        output_form();
    }
    else {
        process_data();
    }
}
else {
    output_form();
}

##</code><code>##

push @errors, "Age not in range" if (!is_integer($age,5,100));
push @errors, "Ugly date format" if (!is_date($date,"YYYY-MM-DD"));
push @errors, "Unknown image type" if (!is_file_type($uploaded_file, "image/jpeg"));