I think you are right, and it's a known bug. However the RT search "helpfully" gave me no results when searching for "given", even though I know of at least one ticket with "given" in the title (unrelated to your question though).

Perl 6 - second systems done right

In reply to Re: Lexical $_ in given/when vs. BLOCK arguments by moritz
in thread Lexical $_ in given/when vs. BLOCK arguments by mbethke

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