How to double numbers left-to-right

doubling.pl

use Test;

sub double($n) {
    my @digits = $n.comb;
    my @result;
    my $last;
    for @digits -> $d {
        my $dd = $d * 2;
        if $dd >= 10 {
            $last++;
            $dd -= 10;
        }
        push @result, $last if defined $last;
        $last = $dd;
    }
    push @result, $last;
    return @result.join;
}

is double(1), 2;
is double(19), 38;
is double(1048576), 2097152;