You must be running an older pugs. In fact, the current (working) version in the pugs repository (r14657) now reads:

sub fib (UInt $n) {
    (state $seen = [0,1,1]).[$n] //= fib($n-1) + fib($n-2);
}
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It would also work as state @seen = 0,1,1 except there's a bug with initializing state variables using a list that causes it to reinitialize every time instead of just the first time, which, while it lets it run, kinda defeats the memoization...