input: 
  def perform_subtitle_operations(releases, operation, engines=[DEFAULTS['subtitle_engine']], lang=DEFAULTS['subtitle_language']):
      """
      This function performs various subtitle operations such as downloading or checking the existence of subtitles for given movie or TV show releases.
      
      >>> perform_subtitle_operations(['path/to/release1'], 'download')
      "[ {'path/to/release1': {'subtitle1.srt': 'Download Success', 'subtitle2.srt': 'Download Success'}} ]"
      
      >>> perform_subtitle_operations(['path/to/release2'], 'exist')
      "[ {'path/to/release2': {'subtitle1.srt': 'Exist', 'subtitle2.srt': 'Does Not Exist'}} ]"
      """
  
output:     results = []
      for release in releases:
          release_result = {}
          for engine in engines:
              if operation == 'download':
                  # Simulate downloading subtitles
                  subtitles = {'subtitle1.srt': 'Download Success', 'subtitle2.srt': 'Download Success'}
              elif operation == 'exist':
                  # Simulate checking existence of subtitles
                  subtitles = {'subtitle1.srt': 'Exist', 'subtitle2.srt': 'Does Not Exist'}
              else:
                  raise ValueError("Unsupported operation")
              release_result[release] = subtitles
          results.append(release_result)
      return str(results)