if ($INPUT{'fourthing'} eq "four") {
$address = "four.txt";
}  elsif ($INPUT{'fourthing'} eq "five") {
$address = "five.txt";
}    elsif ($INPUT{'fourthing'} eq "six") {
$address = "six.txt";
}    else  {&formula; }
[download]

But alas $address remains undefined.

Age-old debugging technique I learned in BASIC: use print statements. In particular, have your script print out the value of $INPUT{fourthing} someplace (perhaps inside <!-- -->) and maybe that will clue you in why it isn't eq to any of your criteria.

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
[download]