There are basically two ways to implement that.

The first method notes that Bool is just an enum, and you can just as easily define your own enum enum TristateBool <False Undecided True>; and override the functions that now coerce to Bool to then coerce to TrisateBool.

Note that there's virtually nothing in Perl 6 that a builtin type can do that a user defined one can't - it's just a matter of effort.

The second method involves Junctions and "normal" Bools, by just saying that $undecided = any(True, False) and have Perl 6 working it out (it might involve some more tweaking, depending on what you actually want to achieve)