The problem is this definition of SvNVX from sv.h:

/* Need -0.0 for SvNVX to preserve IEEE FP "negative zero" because
   +0.0 + -0.0 => +0.0 but -0.0 + -0.0 => -0.0 */
#  define SvIVX(sv) (0 + ((XPVIV*) SvANY(sv))->xiv_iv)
#  define SvUVX(sv) (0 + ((XPVUV*) SvANY(sv))->xuv_uv)
#  define SvNVX(sv) (-0.0 + ((XPVNV*) SvANY(sv))->xnv_u.xnv_nv)
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Their attempts to "preserve IEEE FP "negative zero"", performs + -0.0, which will *always* force SNaN to QNaN.

That said, there is conditional code for the 'F' and 'D' templates in pp_pack.c that bypasses SvNV (which uses SvNVX() if SvNOK is set) in favour of sv_2nv(), which ought to kick in on mingw builds:

#ifdef __GNUC__
        /* to work round a gcc/x86 bug; don't use SvNV */
        anv.nv = sv_2nv(fromstr);
#else
        anv.nv = SvNV(fromstr);
#endif
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But that conditional is not present in 'd' template.

And in any case, sv_2nv() goes right ahead and uses SvNVX() if SvNOKp is set anyway:

Perl_sv_2nv_flags(pTHX_ SV *const sv, const I32 flags)
{
    dVAR;
    if (!sv)
    return 0.0;
    if (SvGMAGICAL(sv) || SvVALID(sv) || isREGEXP(sv)) {
    /* FBMs use the space for SvIVX and SvNVX for other purposes, and 
+use
       the same flag bit as SVf_IVisUV, so must not let them cache NVs
+.
       Regexps have no SvIVX and SvNVX fields.  */
    const char *ptr;
    if (flags & SV_GMAGIC)
        mg_get(sv);
    if (SvNOKp(sv))
        return SvNVX(sv);
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so I don't quite see what purpose the conditional code above achieves ?

Bottom line: There is a bug -- or a combination of bugs -- that is preventing you getting your hands on an SNAN; but I don't think that it will be a quick fix.

Your best bet if you want to 'cure' the problem yourself is to bypass the Perl macros completely. The following won't work pre-5.10, and may not work on some later builds. (The latest version of 32-bit perl I have with inline installed is 5.8.9 and it doesn't work there.) And remember, if you touch the returned value with pack it will probably screw with it again. :

#! perl -slw
use strict; use Config;
use Inline C => Config => BUILD_NOISY => 1, CCFLAGS => $Config{ccflags
+}." -DDEBUG=1";
use Inline C => <<'END_C',  NAME => 'ICexample', CLEAN_AFTER_BUILD =>0
+;

static const unsigned __int64 i = 0x7ff0000000000001;

SV*get_snan() {
    SV *nv = newSVnv(0.0);
    *(unsigned __int64*)&( ((XPVNV*) SvANY(nv))->xnv_u.xnv_nv ) = i;
    return nv;
}

END_C

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

my $snan = get_snan();

printf "%f\n", $snan;

print doubleToHex( $snan );
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