$unitPrice = 10.0;
$discount = 0.175;
$units = 10_000;

$totalPayable = int( $unitPrice * ( 1 - $discount ) * $units ); ## Correct way
printf "Total: £%.2f\n", $totalPayable / 100;

##Total: £825.00

$totalPayable = int( $unitPrice * ( 1 - $discount ) ) * $units; ## WRONG!
printf "Total: £%.2f\n", $totalPayable / 100;
## Total: £800.00