I believe it works like this:-

     $gene->display_id     # Invoke the display_id
                           #   method of $gene object
    [                 ]    # construct anonymous array
                           #   around returned value(s)
  @{                   }   # dereference same
 "                      "  # Interpolate into double-
                           #   quoted string
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although I'm sure others will correct me if I've got it wrong!