A Fractran program potentially returns an infinite list, and infinite lists are a common data structure in Raku. The limit is therefore enforced only by slicing the infinite list.
Extra credit: We can weed out all the powers of two into another infinite constant list based on the first list. In this case the sequence is limited only by our patience, and a ^C from the terminal. The .msb method finds the most significant bit of an integer, which conveniently is the base-2 log of the power-of-two in question.