http://www.perlmonks.org?node_id=947

Unless statements allow you to say unless something is true do this. For example:
$value=20;
unless($divisor==0){      #so long as the $divisor isn't equal to 0
  $value=$value/$divisor; #go ahead and divide $value by $divisor
} else {
  $divisor=1;             #otherwise set $divisor to 1
}
If you've got that I suggest you take a look at while loops