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print('$|--: '); $|=0; for (1..10) { print($|--, ' '); } print("\n");
print('--$|: '); $|=0; for (1..10) { print(--$|, ' '); } print("\n");
print('$|++: '); $|=0; for (1..10) { print($|++, ' '); } print("\n");
print('++$|: '); $|=0; for (1..10) { print(++$|, ' '); } print("\n");

$|--: 0 1 0 1 0 1 0 1 0 1
--$|: 1 0 1 0 1 0 1 0 1 0
$|++: 0 1 1 1 1 1 1 1 1 1
++$|: 1 1 1 1 1 1 1 1 1 1

$,=", ";
$\="\n";
...

$t = 0;
print grep $t^=1, @list;  # a, c

$,=", ";
$\="\n";
...
$n = 3;
$t = $n-1;
print grep !($t=($t+1)%$n), @list;  # a, d