[perl #130610] [BUG] Triangle-reduce operator `[\ ]` unexpectedly flattens each result

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# New Ticket Created by  Sam S. 
# Please include the string:  [perl #130610]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=130610 >


The normal reduce meta-operator doesn't flatten anything:

    ➜  say [,] <a b>, <c d>, <e f>;
    ((a b) (c d) (e f))

But the triangle-reduce operator flattens each of the intermediate
results it returns:

    ➜  .say for [\,] <a b>, <c d>, <e f>;
    (a b)
    (a b c d)
    (a b c d e f)

The result I expected for that last statement, is:

    ((a b))
    ((a b) (c d))
    ((a b) (c d) (e f))