Why not this?

    m[
      <%        # Opening delimiter.
      (?:       # Match stuff that isn't a closing delim:
        [^%]      # Can't start one.
      | %(?!>)    # Might start one but isn't one.
      )*        # As many non-closing-delims as you like.
      %>        # Closing delimiter.
    ]x
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