It is a matter of taste (and style), but I only use unless without the else-clause. If you have the else, then you could just as well use a normal if and get better readability - it's more like waht everyone is used to.

But in a standalone if (!(some_complicated_expression)) the use of unless lets you remove one pair of parenthesis and lets the poor smuck reading the code have one less thing to worry about.

I would rewrite your example as

$value=20;
if ($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
}
[download]

Now the statement says the same as the comment (in your code it should have been #unless the $divisor isn't equal to 0 - 'so long' indicates a repetition IMHO).

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