Unexpected argument processing by MAIN()

Scratching my head over this:

multi sub MAIN(Str:D $arg, Bool :$switch, *@the-rest) {
    say $arg;
    say $switch;
    say @the-rest;
}

When called with: blah-command arg1 --switch

It prints:

arg1
(Bool)
[--switch]

I expected:

arg1
True

What I'd ultimately like to do is this:

blah-command arg1 --switch --unknown-stuff blah --more-unknown-stuff

result in:

arg1
True
--unknown-stuff blah --more-unknown-stuff