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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In reply to Re^3: How will you use state declared variables in Perl6? by TimToady
in thread How will you use state declared variables in Perl6? by Limbic~Region

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