I Believe that P4 has local (my) subroutines now. Also, I always find it ugly to pass $_ explicitly. So that $by_n sub and the subsequent map could be written:

    my sub by_n ($n := $_) { my $k = 0; return -> $x { $x ?? $k += $n 
+:: $k } };
    @list = @list.map( &by_n );
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Come to think of it, I think there's an in-place map operator, too:

    @list.=map( &by_n );
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