This may have something to do with perl's buffering, in particular, when instead of while (<>) I wrote this:

        my $q;
        $q=<STDIN>;
        print "Received(child $$): $q\n";
[download]

When I traced what is happening when it hangs, it looks like parent is waiting waitpid(..) (close($fd) on pipe is supposed to do that), while child blissfully unaware is running read...

Now I can safely go and try to understand merlyn's code.