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, ...
    }
}

##</code><code>##

while(@array1 or @array2) {
    my $i = 0;
    ONE: while(@array1 and $i < $some_large_number/10) {
        $i++;
        my $item = shift @array1;
        # Do various things (as above)
    }
    $i = 0;
    TWO: while(@array2 and $i < $some_large_number) {
        $i++;
        my $item = shift @array2;
        # Do various things (as above)
    }
}

##</code><code>##

while(@array) {
    my $item = shift @array;
    if (used_to_be_in_array1($item)) {
        # Do various array1ish things with $item, some of which will _possibly_: push @array, ...
    } elsif (used_to_be_in_array2($item)) {
        # Do various array2ish things with $item, some of which will _possibly_: push @array, ...
    } else {
        warn $item;
    }
}