Try inserting this at the top of your code.

use strict;
[download]

It will catch barewords like these and a host of other gotchas.

Update: Added requested explanation of why/how atoi.

strict is your good friend who will keep you out of a lot of trouble.

atoi is the C language function ascii to integer which perl uses internally to convert strings to numbers.

You had a - in your code so Perl converted both arguments to numbers (hence the atoi) and then subtracted them and saved the result in $foo.

Hope that makes it clearer.