my %AVSo_by_sig;

sub cid_date {
    @cid_and_date = split /::/, $_[0], 2;
    return "@cid_and_date";
}

sub avs {
    return (split /::/, $_[0], 3)[2];
}

# create a hash of cid_date->elem for the old elements

for (@AVSo) {
    $AVSo_by_sig{cid_date($_)} = $_;
}

# compare new elements to old via their cid_dates

for my $new (@AVSn) {
    my $old = $AVSo_by_sig{cid_date($new)};
    if ($old && avs($old) ne avs($new)) {
        # changed from old to new
        print $new;
    }
}