use strict;
use warnings;

use Inline C => Config =>
  BUILD_NOISY => 1;

use Inline C => <<'EOC';

int uv_fits_double1( UV arg ) {
  if(arg == (UV)((NV)arg)) return 1;
  return 0;
}

int uv_fits_double2(UV arg) {

  while(!(arg & 1)) arg >>= 1;
  if(arg < 9007199254740993) return 1;

  return 0;
} 

EOC

my $ls = 63;
my @in = ( 18446744073709549568, 18446744073709139968);

for(@in) { print "$_: ", uv_fits_double1($_),uv_fits_double2($_), "\n"; }

__END__

With Windows Server 2003 SP1 Platform SDK, this outputs:
18446744073709549568: 01
18446744073709139968: 01

Base 2 representations of the 2 Uvs is (resp):
1111111111111111111111111111111111111111111111111111100000000000
1111111111111111111111111111111111111111111110011011100000000000

showing that both are exactly representable by a double.