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;
    }
}