Learning Perl, 5th Edition

10.2. The until Control Structure

Sometimes, you'll want to reverse the condition of a while loop. To do that, use until:

until ($j > $i) { $j *= 2; }

This loop runs until the conditional expression returns true. It's really just a while loop in disguise, except it repeats as long as the conditional is false, rather than true. The conditional expression is evaluated before the first iteration, so this is a zero-or-more-times loop, just like the while loop.^[*] As with if and unless, you could rewrite any until loop to become a while loop by negating the condition. But generally, you'll find it simple and natural to use until from time to time.

^[*] Pascal programmers, take note: in Pascal, the repeat-until always runs at least one iteration, but an until loop in Perl may not run at all if the conditional expression is true before the loop starts.