I usually do something like this (pseudo-code != Perl):

if (data_submitted()) {
    @errors = validate_data();
    if (@errors) {
        output_errors(@errors);
        output_form();
    }
    else {
        process_data();
    }
}
else {
    output_form();
}
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Forms appearence, data validation and processing are VERY application specific, so I don't think that you might cheat a lot there. On the other hand, the validate_data() sub can win a lot from simplified validations. I'd say that it would be very helpful if I could do something like this (pseudo-code again):

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"));
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Leonid Mamtchenkov