Two possibilities.

1. Convert the I64 to a double and return it as an NV:

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

2. Return the 8 bytes packed into a string and unpack them in Perl:

   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 ];
   [download]

Either way, if you are using a 32-bit Perl, you are going to be restricted to integers < ~2**52, so the latter method would only really be useful if you are going pass the I64 (back) to another piece of i64 capable code.