In response to your query as to whether or not perl will
allow you to check the results of your form before submittal,
no it will not. Unlike JavaScript which is a client side
programming language, in which you can instruct the client to
verify the contents of a form before submitting it, perl is
restricted to server-side processing. This means that perl
can only verify the contents of your form after it has been
submitted.
This is not entirely true. In your HTML output you can embed PerlScript which is a perl client side scripting lang. You can use the PerlScript to verify and send it to the server side script.