I just added an example to pugs which uses state for memoization.

use v6;

my $n = @*ARGS[0] // 42;
say fib($n);

sub fib (Int $n) {
    state %seen;
    return 1 if $n < 2;
    %seen{$n - $_} //= fib($n - $_) for 1 .. 2;
    return %seen{$n - 1} + %seen{$n - 2};
}
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Hmm, why a hash? And why not optimize assuming the current value is already in @seen? Also, declarators need not be separate statements. So I'd probably write it like this:

    sub fib (Int $n) {
        return 1 if $n < 2;
        state @seen[$n] //= fib($n-1) + fib($n-2);
    }
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I think that may be a little clearer as well. If I wanted to golf it just a bit more I'd say:

    sub fib (Int $n) {
        $n < 2 or state @seen[$n] //= fib($n-1) + fib($n-2);
    }
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If I wanted to golf it a lot more I'd say:

    my&fib:={$_<2or state@s[$_]//=fib($_-1)+fib $_-2}
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Um. Er...?

pugs>sub fib (Int $n) { return 1 if $n < 2; state @seen[$n] //= fib($n
+-1) + fib($n-2); }
Internal error while running expression:
***
    unexpected "["
    expecting word character, "::", "=", ":=", "::=", ";" or "}"
    at <interactive> line 1, column 52
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Update: This works though

pugs>  sub fib (Int $n) { state @seen; return 1 if $n < 2; @seen[$n] /
+/= fib($n-1) + fib($n-2); };
undef
pugs> say fib 4;
5
undef
pugs> say fib 8;
34
undef
pugs> say fib 800;
1121023813016570197539221312040081070329432498024398917379911096096424
+176870242714672419719090010009284331740160122026805305229708722215290
+03044406006693244742562963426
undef
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---

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In the absence of evidence, opinion is indistinguishable from prejudice.

My point was that you'd use standard memoization for that, not your own mechanism. In other words: state will (as far as I can estimate) not be used much for memoization, because it's easier to just say "is memoized" or something alike.

Juerd # { site => 'juerd.nl', do_not_use => 'spamtrap', perl6_server => 'feather' }

That's currently specced as "is cached" though I don't believe it's implemented yet. However, even after it's implemented, it's not going to be smart enough from the type signature to know whether an incoming parameter is sparse or dense, and for something like fibonacci an array is going to provide a faster memo cache than a hash is. Maybe the memoizing code can be smart enough to figure that out heuristically, but that's not a sure thing at this point.