I'm also thinking it would be nice if one could also write
my %Mode : Field(rw custom);
which would not create an accessor, but would test for the existence of one and complain if it didn't find one. That way the code becomes self documenting. Maybe a do-nothing private attribute would be nice to that end as well.
I'd probably avoid this since it's not something that you can guarantee. You could have an accessor/mutator - but with a different name. You could also have a method that wasn't an accessor/mutator with the same name. In either case the "custom" documentation would end up being inaccurate.
Actually, I was originally going to suggest some syntax that lets one define their own accessor code. Unfortunately it seems not to be possible to pass anything but strings to the attribute routine, so passing a coderef is out.
But I guess the generator can check whether a method with the mutator's/accessor's projected name already exists and skip generation if so. It would be easy to define some extra syntax for telling the attribute the desired names.
# separate methods
my %Foo : Field(r:get_field w:set_field);
# combined mut/acc method
my %Foo : Field(rw:quux);
It would be nice there is some more syntax so the autogenerated accessor can do limited parameter validation without one having to write custom ones for simple cases like 'HASH' eq ref $_1. For more complex cases, the could be something like:
my %Foo : Field(rw prew:bar postw:baz postr:quux);