Thie following code is instructive:

use strict;
use warnings;

$| = 0;
print $|++ . " " for (1..4);

$| = 0;
print "\n";
print $|-- . " " for (1..4);
[download]

Prints:

0 1 1 1 
0 1 0 1
[download]

Why does that happen? Well, from the documentation snippet referred to in Perl Idioms Explained - $|++, you see that the only legal values for $| are 0 and 1. When you increment $| to 2 it is set back to 1, so the 1 'sticks'. However, when you decrement $| to -1 the -1 is changed to 1. Next time it decrements from 1 to 0 - back where you started. A cheap toggle, so long as you are not interested in the effects on buffering.

You might also consider:

use strict;
use warnings;

my $x = 1;
print $x ^= 1 . " " for (1..4);
[download]

which prints:

0101
[download]

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