in .swg
%typemap(in) void (* cb)(void *) {
   $1 = INT2PTR($1_ltype, SvUV($input));
   printf(":: x%x to x%x\n", (unsigned long long)SvUV($input), $1);

 if(SvGMAGICAL($input)) mg_get($input);
 $1 =  DPTR2FPTR($1_ltype, SvUV($input));
 printf("::DPTR2FPTR x%x to x%x\n", SvIV($input), $1);

}