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.

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.