Waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there is no such child process. The status is returned in $?. If you say
use POSIX ":sys_wait_h";
#...
waitpid(-1,&WNOHANG);then you can do a non-blocking wait for any process. Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) system calls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)
See perlipc for other examples.