That's not a "Sieve of Eratosthenes". You're testing with all the numbers between 2 and the square root of the max. Eratosthenes was smarter than that. He knew only to test with the primes. The primes you're generating.

Anyway, your problem is that you remove the prime itself from @n. Don't do that.

my $max = shift || 100;
my @nums = (2 .. $max);
my @primes;

while (@nums) {
    my $prime = shift @nums;
    push @primes, $prime;
    if ($prime ** 2 > $max) {
        push @primes, @nums;
        last;
    }
    @nums = grep {$_ % $prime != 0} @nums;
}

say "@primes";
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