I agree with Russ, don't use a redirect. It is confusing to users, because they won't know what they did wrong.

I usually just take an array of "required" field names and loop through it and any that are blank get added to a scalar that contains an HTML list, like so:

use CGI;

my $q = new CGI;

my $missed;
my @required = qw(name address phone email);
for (@required) {
    $missed .= "<li>$_</li>\n" if $q->param($_) eq "";
}
do {
    print "Content-type: text/html\n\n";
    print qq~
You forgot to fill in the following required fields:
<ul>$missed</ul>
~;
} if $missed;
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