Today I spend time to understand why do I get wrong answers, and I don't know an answer. Later I tried and found how to overcome problem: use bigint and add zeroes. See this:

while(<>){
    ($a, $b)=split/ /;
    print $a - $b, " ";
    print (($a+0)-($b+0), " ");
    use bigint;
    print $a - $b, " ";
    print (($a+0)-($b+0), "\n");
    }
[download]

STDIN:
9 7
900000000000000009 900000000000000007

STDOUT:
2 2 2 2
0 0 0 2
[download]