The subroutine is a signal handler indicating what the process should do when it receives a TERM signal (a signal sent to processes to indicate they should exit), an INT signal (a request to interrupt the process), a PIPE signal (the process attempted to write to a pipe that was not connected at the other end), or a CHLD signal (a child process created by this process exited). For more information on signal handling, look for %SIG in perlvar and at perlipc under 'Signals'.

Definitions of those types of signals on wikipedia...

• SIGTERM
• SIGINT
• SIGPIPE
• SIGCHLD