That typically happens somewhere around 9e15

The resolution typically is 53 bits, as customary in IEEE floats.

print 2**53;
__END__
9.00719925474099e+015
[download]

and on my system:

$\ = "\n";
my $x = 2**53;
my $y = 2**53+1;
my $z = 2**53-1;
print $y-$x;
print $x-$z;
__END__
0
1
[download]

So that is indeed the threshold here. (ActivePerl 5.8.4 on Win32)