$inval = pack("d*",1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)
$results = pack("d*",(0)x15)
$summary_result = c_function($inval,$results,15)
@real_results = unpack("d*",$results)

use Inline C => <<'END_OF_C_CODE';

double c_function(char * the_data, char * results,int nelems) {
    double *real_data = (double *) the_data;
    double *real_results = (double *) results;
    double summary_result;
    /*  do your fast calculation here, storing elements in *real_results  */
    return summary_result;
}
END_OF_C_CODE