Documented in For Loops in perlsyn:

As a special case, if the test in the for loop (or the corresponding while loop) is empty, it is treated as true. That is, both

        for (;;) {
            ...
        }
[download]

and

        while () {
            ...
        }
[download]

are treated as infinite loops.

Not very well placed documentation, and counter-intuitive from my perspective, but documented.

Update:: A little Googling pulled up while () vs. while (1), particularly Re: while () vs. while (1).