[perl #118053] cannot specify optional positional with Mu as default

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# New Ticket Created by  Elizabeth Mattijsen 
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[18:35:53] <lizmat>      r: sub a ($a=Mu) { }; a
[18:35:54] <+camelia>    rakudo 9c5650: OUTPUT«Nominal type check failed for 
parameter '$a'; expected Any but got Mu instead␤  in sub a at 
/tmp/IvMlnzFJOY:1␤  in block  at /tmp/IvMlnzFJOY:1␤␤»
[18:36:18] <lizmat>      expected Any?  but but, I specified Mu as the default