Thank you very much for your pointers. The problem was solved by following your suggestions. The new code looks like:

   while (! eof(RCMD))
   {
      my @fds = $sel->can_read(5);
      #print NSY "FDS = @fds\n";
      if ($#fds == 0)
      {
         my $in = <RCMD>;
         print $in;
      }
      else
      {
         my @fds = $wsel->can_read(0);
         #print NSY "FDS = @fds\n";
         if ($#fds != 0)
         {
            #print NSY "Closing as stdout closed\n";
            exit(0);
         }
      }
   }
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