choroba has provided a good solution. Depending on what you need exactly, this is another possible solution using a formatter (demonstrated here on the REPL): 
> my $fmt = { sprintf "%04d-%02d", .year, .month };
-> ;; $_? is raw { #`(Block|173600432) ... }
> say Date.new(2019, 1, 1, formatter => $fmt);
2019-01

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Update: Please note that you don't need to predefine the formatter: 
> say Date.new(2019, 1, "1", formatter => { sprintf "%04d-%02d", .year
+, .month })
2019-01

[download]

In reply to Re: strptime("%Y-%m") in perl6 by Laurent_R
in thread strptime("%Y-%m") in perl6 by leszekdubiel

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