It sounds exactly like a trivial bug in a module that overloads subtraction. But it doesn't appear to me that XML::Smart does that.

And the problem is with the $last_status_data->{value} result. You seem to me to strongly imply that $now_value is coming from XML::Smart, but I find no clear indication as to what type of object $last_status_data is. That is where you need to look.