For my own sake, and others I hope, why do you have to do ~~ rx:perl5/([XO])\1\1/ instead of just plain old =~ /([XO])\1\1/?

The old pattern-match-binding operator =~ has been usurped by the smart match operator ~~, and "regular expressions" have been reworked in Perl6 to "rules". The old regular expression engine will still be there, but you have to use the vaguely ugly rx:perl5// syntax, or rx:P5// will work too (I think). The reason he used a P5 regex in the code is because Pugs doesn't have Perl6 rules support yet.