Idiomatic? Needlessly obfuscated, I'd say, without even a gain
in keystrokes. $|=1 takes four keystrokes, $|++ as well, but has more shifted keys. And it gives the false impression that you can turn buffering off by doing
$| -- as often as you've given $| ++.
$| = 0;
$| ++; $| ++; $| --; $| --;
print $|; # prints 1, not 0!
I would say both. Part of me dislikes the peculiar ++ behaviour, but in it's defence, it is idiomatic.
Come to think of it, this is another of those cargo cults, isn't it? People doing $|++ without thinking of the consequences, such as non-nestability. If you know that there will not be any nested $| accesses for the same handle, then it's ok.
A minor point, ++ is easier to type than =1 in this case, as you already have your finger on the shift key from the |, so it requires two less actions (release Shift, move finger across keyboard to 1).