actually for one, on a linux box, it would be

kill -9 $pid

that is assuming you are calling the system's kill function and not a sub.
kill 9 $pid will attempt to literally kill pid 9 and not kill $pid with a signal of 9

Update: as I find by doing the research, perlfunc:kill and perlman:perlipc show that my suggestion sucked, using kill with -9 actually works contrary to the shell command, and kills the process group.

hmmm, now I know there's a kill() function.