See the current Perl documentation for kill.
Here is our local, out-dated (pre-5.6) version:
kill - send a signal to a process or process group
Sends a signal to a list of processes. The first element of the list must be the signal to send. Returns the number of processes successfully signaled.
$cnt = kill 1, $child1, $child2; kill 9, @goners;
Unlike in the shell, in Perl if the SIGNAL is negative, it kills process groups instead of processes. (On System V, a negative PROCESS number will also kill process groups, but that's not portable.) That means you usually want to use positive not negative signals. You may also use a signal name in quotes. See Signals for details.