perl -e '
  open X, "</etc/passwd";
  fork();
  while (readline(X)){
    print "$$: $_";
    sleep 1
  }
  print "$$: all done\n"
'

[download]
I was expecting to see every line printed twice (once by each process, as they don't share a file pointer), or at a push, every line printed once-ish, (if they do).

I definitely wasn't expecting one process not to print anything: 
8488: root:x:0:0:root:/root:/bin/bash
8489: all done
8488: daemon:x:1:1:daemon:/usr/sbin:/bin/sh
8488: bin:x:2:2:bin:/bin:/bin/sh
8488: <rest of the file>
8488: all done

[download]
Can someone explain what is going on please? At the start of the while loop I would expect the two processes to be identical, bar the return value of fork... 
$ perl -v

This is perl 5, version 14, subversion 2 (v5.14.2) built for x86_64-li
+nux-gnu-thread-multi
(with 89 registered patches, see perl -V for more detail)

[download]

In reply to confusing fork/readline behaviour by flipper

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