David, Thanks. After further investigation in the XS extension, I see this in

func bless_into_proc
...
      case 'l':  /* long */
    l_val = va_arg(args, long);
    hv_store(myhash, key, strlen(key), newSVnv(l_val), 0);
[download]

Which stores the rss (long) as numeric (float) with implicit casting, so it is still a bit of mystery why negative value shows up?
- Jim