Code which has suddenly slowed down

euler-10.pl6

use v6;
my int $max = 2_000_000;

# find all primes up to $max using The Sieve of Erathostenes
my int @a = (0..$max);

for (2..($max div 2)) -> int $i {
  my int $j = 2;
  @a[$i * $j++] = 1 while $i * $j <= $max;
}

# remove discarded numbers and store the rest in a set for ease of lookup
my @primes = @a.grep(-> int $num {$num > 1});

say @primes.sum;