[perl #125427] [BUG][Feature][GLR] Can't define custom flat prefix op with pipe

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# New Ticket Created by  Brent Laabs 
# Please include the string:  [perl #125427]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=125427 >


07:02     labster     m: multi sub prefix:<|> (\a) { a.flat }; say
(|[1,2,3]).perl
07:02     camelia     rakudo-moar d6430c: OUTPUT«5===SORRY!5=== Error while
compiling /tmp/jvrQI4RBsS␤Arg-flattening | is only valid in an argument
list␤at /tmp/jvrQI4RBsS:1␤------> 3i sub prefix:<|> (\a) { a.flat }; say
(|7⏏5[1,2,3]).perl␤»

Expected, one of:
 A) single pipe works as shorthand for flat outside of arg list
 B) Rakudo allows me to define my own flat operator (though, in this case,
prefix:<|||> does work)

Maybe this should be notabug, but if so I'd like an explanation as to why
neither A nor B should work.