Changing the code as follows:

  unless (open F, "-|")
  {
    open STDERR, ">&=1";
    select(STDIN);
    $| = 1;
    select(STDERR);
    # also tried select(F) here
    $| = 1;

  
    exec "$command";
  }
  
  while (<F>)
  {
    $buffer .= $_;
    $cache->set($session, [0, $buffer]);
  }
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Does not make any difference.

CT