[perl #63742] Rakudo doesn't count $_ in the arity of a block

[Carl Mäsak]

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<moritz_> rakudo: my $x = { $_*2 }; say $x.arity
<p6eval> rakudo 8bbc31: OUTPUT«0␤»
<moritz_> that however is wrong
* masak submits

Expected result: 1. I'm not sure what it takes for a block to
"recognize" that it has a $_ as an implicit parameter, though.