One of the big problems here is that == is used for number equality, not string equality. Use

if ($discount eq '40+15')
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To do what you are attempting in this line.

However, because things may change, this is probably not the best way to approach what you are trying to do. What if someday you need to be able to evaluate '35+15' or some other such thing? If the discounts are always going to appear in this format, you could do something like:

@discount = split(/\+/, $discount);
$discount = 1;
foreach (@discount) {
   $discount*=$_/100;
}
$discount*=100;
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and then do your quantity calculations.

-HZ