I think this excertp from perlipc explains it a bit better than what you have:

           use English '-no_match_vars';
           my $sleep_count = 0;

           do {
               $pid = open(KID_TO_WRITE, "|-");
               unless (defined $pid) {
                   warn "cannot fork: $!";
                   die "bailing out" if $sleep_count++ > 6;
                   sleep 10;
               }
           } until defined $pid;
[download]

update: Just forgot.... You already read it... So here is a bit of explanation. The "open" returns a process ID, 0 or undef.

undef

We are out of processes. This is when open fails

0

This is the child process... The "or" part of your open. But it would be reached for undef too!

pid

This is for the parent process to have a pid to wait for.