I think what's happening is that the process that calls tick() is exiting before the three exec(@ping_cmd) has a chance to return (which has the effect of only some of the run_on_finish() are getting a chance to run).

The callback now runs for ALL child processes by calling $pm->wait_all_childern;

Since I am waiting on all children, I am probably not creating any defunct processes from tick().. which doesn't really matter since not calling wait_all_children had the effect of exiting immediately and the defunct child processes would have been inherited by init. Is this correct?

sub tick {

    my $MAX_PROCESSES    = 1000;
    my $pm = new Parallel::ForkManager($MAX_PROCESSES);

    my @ping_cmd = ("ping", "testhost", "-s 5", "-c 1");

    $pm->run_on_finish(
               sub { 
               my ($pid, $exit_code, $ident) = @_;
               my ($check_id, $host) = $ident =~ /^(.*?) on (.*)/s;
               print "^^^^^^^^^^^run_on_finish: $ident (pid: $pid) exi
+ted with code: [$exit_code] host: [$host]\n";
               }
               );
    
    $pm->run_on_start(
              sub {
              my ($pid,$ident)=@_;
              print "** $ident started, pid: $pid\n";
              }
              );

    for(my $i=0; $i< @servers; $i++){
        my $srv_id    = $servers[$i]{id};
        my $srv_name    = $servers[$i]{name};

        $ping_cmd[1] = $servers[$i]{name};
    
        print "$srv_name:\n";
        
        $pm->start("ping on $srv_id") and next;
        exec(@ping_cmd);
        print(STDERR "ping on $srv_name exec failed: $!\n");
        _exit($!);    
    }
    print "Waiting for all pings...\n";
    $pm->wait_all_children;
    print "Tick finished.\n";
}
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