use CGI;
$q = new CGI;

if ($q->param('foo')) {
    #do stuff if has stuff in field
}else {
   print $q->redirect('http://www.foo.bar');
}


##</code><code>##

if ($q->param('foo')) {
    #do stuff if has stuff in field
}else {
   my $bar = $q->param('bar'); #should test this too.
   print $q->header;
   print "foo was empty please try again";
   print "your html stuff here again";
   #this next line keeps the info already provided.
   print "<input type=text name=bar value=$bar>";

}