I'm not supposed to use unless.

You're not supposed to use unless?? Let me guess, this is homework right?

Any number of the answers you've gotten already will work.

Personally if I'm not going to use unless, I prefer

if (whatever_the_test_for_the_condidtion_is) {
   die("A complete and total disaster happened");

} 

# proceed with your code here
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Note I don't include the useful code inside of an else since that's an extra bracket you've got to keep track of and another layer of indentation which could get screwed up. Of course that's just me, others will prefer to have this extra stuff. Unless it is unless use whatever (assuming there aren't other silly restrictions).