$packed = pack 'd', '7ff0000000000001';
$d = unpack 'h16', $packed;

##</code><code>##

use warnings;
use strict;

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

use Inline C => <<'EOC';

#ifdef _WIN32
typedef __int64 BIGGUN;
#else
/* "long" is 64-bit on my linux box */
typedef long BIGGUN;
#endif

typedef union {
    double  d;
    BIGGUN i;
} BITS64;

void bytes(double d) {
  int i;
  void * p = &d;

  for(i = 7; i >=0; i--)
    printf("%02x", ((unsigned char*)p)[i]);

  printf("\n");
}

double get_snan () {
  BITS64 x;

  x.i = 0x7ff0000000000001;

  bytes(x.d);

  return x.d;
}

EOC

print doubleToHex(get_snan());

sub doubleToHex { scalar reverse unpack 'h16', pack 'd', $_[0] }