while(@array1) {
    ONE: while(@array1) {
        my $item = shift @array1;
        # Do various things with $item, some of which will _possibly_
        # push @array1, ...
        # and/or
        # push @array2, ...
    }
    TWO: while(@array2) {
        my $item = shift @array2;
        # Do various things with $item, some of which will _possibly_
        # push @array2, ...
        # and/or
        # push @array1, ...
    }
}