There are many ways to accomplish this. Here's one (untested):

#!/usr/bin/perl -wT
use strict;
use CGI qw/:standard/;

my %errors;
my ( $candidate ) = ( param( 'candidate' ) =~ /^([\s\w]+)$/ );

# candidate must be defined and have at least one non-whitespace chara
+cter
if ( ! defined $candidate or $candidate !~ /\S/ ) {
    $errors{ candidate } = 'You must supply a value for candidate';
}

# grab more params and untaint

if ( %errors ) {
    display_page_with_errors( \%errors );
}
[download]

Cheers,
Ovid

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-T being one of the more important things you can learn doing web stuff.

It is helpful to the user if you send down JavaScript to the browser that will test this before sending the form up to the server.

Important:
Do not trust this as a final verification of the input. For anything that you think is important, you must do the final check on the server. Nevertheless, it is a good approach to let the user struggle with your form and get it pre-approved by your JavaScript before sending it through the 'net and bothering your server.

Once you detect anything invalid at the server level (in your CGI script) you have no other tool than sending a full HTML page with your error messages down to the browser client.

f--k the world!!!!
/dev/world has reached maximal mount count, check forced.