gistfile1.txt

sub simple-moving-average(Int $P where * > 0) {
    -> Real $x {
        state @a;
        @a.push($x);
        @a.shift if @a > $P;
        ([+] @a) / @a;
    }
}

my &ma3 = simple-moving-average(3);
say &ma3(1);
say &ma3(1);
say &ma3(2);
say &ma3(3);

You don't even need the & sigil to invoke the sub -- this works too:

say ma3(1);