Just a guess, but maybe bkill is returning a negative exit code in case of "Job has already finished", which might then indirectly trigger this warning from win32.c:

if (status < 0) {
    if (ckWARN(WARN_EXEC))
        Perl_warner(aTHX_ packWARN(WARN_EXEC), "Can't spawn \"%s\": %s
+", argv[0], strerror(errno));
    status = 255 * 256;
}
[download]

The reason (errno) is presumably just a random value left over from some previous error...

(status is the return value of win32_spawnvp(), so to really understand what's going on, one would have to take a closer look to figure out what exactly that routine is returning under what circumstances... (which I haven't done yet))