3.2.190 waitpid

waitpid PID, FLAGS

This function waits for a particular child process to terminate and returns the pid when the process is dead, or -1 if there are no child processes, or 0 if the FLAGS specify non-blocking and the process isn't dead yet. The status of the dead process is returned in $?. To get valid flag values say this:

use POSIX "sys_wait_h";

On systems that implement neither the waitpid(2) nor wait4(2) system call, FLAGS may be specified only as 0. In other words, you can wait for a specific PID, but you can't do it in non-blocking mode.