This is derived from an example in the Perl Developer's Dictionary and it's a cool trick if you can stomach it. 

$a=0;
$_=0;
while($_++<10) {
        if (! $a..(!$a)) {
                print "True!\n";
        } else {
                print "False!\n";
        }
        $a++;
}

[download]
The if with the .. operator there returns true exactly once (so long as $a isn't externally modified) for the entire life of the program.

You can make the example a bit more concise using this: 

$a=0;
$_=0;

while($_++<10) {
        if (! $a..(!$a++)) {
                print "True!\n";
        } else {
                print "False!\n";
        }
}

[download]
But using an auto-increment in an expression where the target variable appears twice makes me nauseous. Probably indigestion from my C days.

In reply to Bi-stable operator as a true-once value. by clintp
in thread A TRUE-once variable by gmax

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