another way is to use shared memory to have same value visible from each parent and childs process.

this is an example:

$ipckey = IPC_PRIVATE; 
$idshm = shmget( $ipckey, 200, 0666 ) || die "\nCreation shared memory failed $! \n";

$statusproc = "00000";
shmwrite( $idshm, $statusproc , 0, 100 ) || die "shmwrite $!";

if ( $pid == 0 )
{
       # do what I  have to do .............
      # end of task
       $statusproc = "1";
       shmwrite( $idshm, $statusproc , 0, 1 ) || warn "\n\n shmwrite $! \n";
}
else
{
    ................
         # check statusproc  from time to time 
        shmread( $idshm, $statusproc , 0, 1 ) || warn "\n\n shmread $! \n";
         if(  $statusproc  eq "1" )  # the child has ended it's task