Regarding decrementing IV_MIN(Integer minimum)

[Geeta Kathpalia, Noida]

Hello all
 
        I am executing the following perl code(using 5.8.0):
 
                $a = -9223372036854775808;
                $a--;
                if ($a ==  -9223372036854775807)
                {
                        print "ok \n";
                }
        
         The output which i get is that:     ok get printed.
 
How is it possible??
 
Thanks & Regards
geeta