$x = 2 ** 45 + 1;
$y = 2 ** 45 + 1.5;

if($x + .5 == $y) {print "fine\n"}
else {print "crap\n"}

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

# Then
$x = 1e55 + 1e44;
$y = 1e55 + 2e44;
# ...