Doesn't this work for you?:

$d = unpack 'd', pack 'Q', 0x7ff0000000000001;;

print $d;;
1.#SNAN

printf "%f\n", $d;;
1.#SNAN0
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If inline C has to be involved, do it this way:

#! perl -slw
use strict; use Config;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'snanic', CLEAN_AFTER_BUILD =>0;

#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;

SV*get_snan() {
    BITS64 x;
    x.i = 0x7ff0000000000001;
    return newSVnv( x.d );
}

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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Prints:

C:\test>snan-ic.pl
1.#SNAN0
7ff0000000000001
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