$sum = 0; 
$sum += $_ for (0)x128, (255)x128; 
print $sum, "\t", $mean = $sum / 256;;
32640    127.5

$SoS = 0;  
$SoS += sqrt( ( $_ - 127.5 )**2 ) for (0)x128, (255)x128; 
print $SoS, "\t", $stdDev = $SoS / 256;;
32640    127.5