[perl #126890] [BUG] pair with Nil key deparses incorrectly

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$ perl6 -e '((Nil)=>1).key.WHAT.say; ((Nil)=>1).perl.EVAL.key.WHAT.say'
Nil
(Str)

The problem is that the .perl of that pair is "Nil => 1", which parses
as a string key rather than the Nil object.  Parens or some other
circumlocution is required in order to preserve the meaning of "Nil"
in this context.

-zefram