It can be both .. and .... They have subtly different effects.

With two dots, it's possible for both the start and end checks to be true on the same line. This means that the operator goes from false to true and back to false again on one evaluation. With three dots, if the start check is true, then the end check isn't checked until the next evaluation - thus forcing at least one iteration with the operator returning true.

"Perl makes the fun jobs fun
and the boring jobs bearable" - me