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;