I'm surprised to see .pm6 extension still being used, at least on the rakudo documentation page. Do those contain Raku code, or Perl?

These would contain Raku code. But as with Perl, the core developers value backward compatibility much. Since many versions of Raku in use do not know of the new .rakumod extension yet, the old extension is used for many modules still.

This is probably going to change in the next language revision of Raku. See https://github.com/Raku/problem-solving/blob/master/solution... for the nitty gritty.