It seems to be ok if shutdown is used instead of remove and close.

for $h ($mux->handles()) {
  if ($fhs->{$h}{object}{user}{_id} == $id) {
    $mux->shutdown($h, 2);
    last;
  }
}
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One is not supposed to modify arrays while they are being iterated over. There was problem because freed value used in iteration in $mux->loop, so the perl saw freed value and segfault.