How does --$| work in Perl?

Question

Recently I came across this way to filter out every second value of a list:

perl -E \'say grep --$|, 1..10\'
13579

How does it work?

Answer 1

The point is this use is just a nasty hack.
$| ( or his more readable alias $OUTPUT_AUTOFLUSH) is a special variables to control the autoflush of STDOUT ( or the current selected filehandle). Therefore it only accepts true (1) or false (0).