If you need the product, why are you expanding your string at all? You should then be able to calculate your product by simply multiplying the letters by their factors in front. But if I interpret what you say that way, I still don't get your result:

perl -le "print 2*1.5*(50/100)*2*2.5*(55/100)*2*3.5*(60/100)"
17.325
[download]

Please explain what calculation you want to perform.

The structure of if..elsif is a very cumbersome way to approach your problem, so I doubt that tye recommended it as a solution for this specific problem.