sub localised_hashvalue {
    package localised::hashvalue;
    my ($ref, $key, $value) = @_;
    my $exist = exists $ref->{$key};
    my $old   = $ref->{$key};
    $ref->{$key} = $value;
    return bless [$ref, $key, $exist, $old];
    sub DESTROY {
        my ($ref, $key, $exist, $old) = @{+shift};
        $ref->{$key} = $old;
        delete $ref->{$key} if !$exist;
    }
}