I think, that if you rewrite this with sysread/syswrite, or, like in merlyn's solution with length-based communication, it starts working.
This may have something to do with perl's buffering,
in particular, when instead of while (<>) I wrote this:
print "Received(child $$): $q\n";
it stopped hanging on close($fd)
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.