Im not familiar with setting

$| = undef
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My understanding was that there are only 2 buffering behaviours, and they are based on the truth value of $|
So, $| = undef would evaluate to false, the same as the default value of $| = 0
If $| is true, the output buffer is flushed after each output operation (print, printf, write). If $| is false, block buffereing occurs.
What am i missing?

Update:fixed a minor typo - i had $\ in place of $| in one place.