my $errstr = '';

for my $param( qw( name email phone url requests ) ) {
    $errstr .= "Please enter your $param<br>\n" unless defined param($param);
}

# if $errstr is true we have some errors, so tell user
if ( $errstr ) {
    print "Content-Type: text/html\n\n";
    print "<p>Please click back and correct these error(s):\n<p>" . $errstr;
  exit 0;
}

# got data OK so continue processing