Add Mu.perl back

... which may have been removed by mistake.

Make its documentation somewhat minimal.

Author: stoned

doc/Type/Mu.pod6

@@ -169,16 +169,13 @@ $something> and C<say $something.gist> generally produce the same output.
     say Mu.gist;        # OUTPUT: «(Mu)␤»
     say Mu.new.gist;    # OUTPUT: «Mu.new␤»
 
-=head2 method raku
+=head2 method perl
 
-    multi method raku(Mu: --> Str)
+    multi method perl(Mu:)
 
-Returns a Raku-ish representation of the object (i.e., can usually be
-re-evaluated with L<EVAL|/routine/EVAL> to regenerate the object). The exact
-output of C<raku> is implementation specific, since there are generally many
-ways to write a Raku expression that produces a particular value. Since the
-change of name to Raku, this method is deprecated and might disappear in the
-near future. Use C<.raku> instead.
+Calls L«C<.raku>|/routine/raku» on the invocant.  Since the change of the
+language name to Raku, this method is deprecated and might disappear in
+the near future. Use C<.raku> instead.
 
 =head2 method raku