double i64d ( ) {
    __int64 x = 123435;
    return (double)x;
}

##</code><code>##

SV* i64p( ) {
    union {
        __int64 i;
        char s[8];
    } x;
    x.i = 123456789;
    return newSVpv( x.s, 8 );
}

...

## Note: The unpacking will vary with endianness of the platform.
my( @i64 ) = unpack 'VV', i64p();
print $i64[ 1 ] * 2**32 + $i64[ 0 ];