$| is magic. It is always either 0 or 1. $|++ sets its value to 1 no matter what its original value was.

However, $|-- is a bit trickier. $|-- toggles the value of $|. So, its actually a lot like $| = !$_

% perl -le 'print $| and $|++ for 1..5;'
0
1
1
1
1

% perl -le 'print $| and $|-- for 1..5;'
0
1
0
1
0
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-Blake