I think it should be somewhat obvious that $sumValue must be integral unless $firstValue and $secondValue are not ... or if their sum is too high to be represented in your native integer types.

So, I'd suggest some debugging techniques, such as printing out the $firstValue and $secondValue before setting $sumValue, to make sure they are what you think they are, and to check if they are about to wrap the native integer type for your platform. e.g., if you're using 32-bit perl, chances are that the maximum integer is about 4 billion (2**32-1), while if you're using 64-bit perl, or 32-bit perl with use64bitint defined, the maximum is about 4 billion squared (2**64-1).

Of course, I'd also check what $sumValue was right before sending it to setValue. Never hurts to double-check.