Perl5 リファレンス

       wait    Behaves like the wait(2) system call on your
               system: it waits for a child process to terminate
               and returns the pid of the deceased process, or
               `-1' if there are no child processes.  The status
               is returned in `$?'.  Note that a return value of
               `-1' could mean that child processes are being
               automatically reaped, as described in the perlipc