ikegami is right (as usual). On top of what he said, some checking of values would be a good idea, to avoid unsightly crashes (like "divide by zero"). Suppose that you just fix the assignment to @product (using "push" as suggested in the first reply). Then run the script like this:

$ your_script
^D
[download]

(or "^Z", if you're using MS-windows/DOS-Prompt -- cmd.exe or command.exe -- instead of a linux/unix/macosx shell / bash for windows)

What happens?

My point is: check what $num is before you use it for division.