Is "use bignum;" an option? --->

use bignum;
use warnings;

$x = 2**54 + 123; # 18014398509482107
$y = 2**54 + 125; # 18014398509482109

if($x + 2 == $y) {print "fine\n"}
else {print "crap\n"}

if($x == $y) {print "crap\n"}
else {print "fine\n"}

--->
fine
fine