You have several options besides do. You could use parenthesises instead, like this:

say join "\n", grep *.day-of-week == 5, map { Date.new: |$_, 1 }, (1900..2019 X 1,3,5,7,8,10,12);

In this case I just thought the code looked better without parenthesises, so I chose to use do instead. The docs has a couple of sentences about this option here.

BTW, I chose the form Date.new: blah — the colon variant — instead of Date.new(blah) also because I wanted to avoid parenthesises. This freedom to chose is the essence of Perl’s credo “There’s more than one way to do it” I guess.

Rant: This freedom of choice is the best thing with Perl 6, but it has a downside too. Code can be harder to understand if you’re not familiar with the variants another programmer has made. This is a part of the Perls’s reputation of being write-only languages.

It won’t happen, but personally I’d like to see someone analyse real-world Perl 6 code (public repositories on Github for instance) and figure what forms are used most. The analysis could have been used to clean house — figuring out what forms to keep and which should be removed. Perl 6 would become a smaller — and arguably easier — language while staying just as powerful as it is today.