my $cookie_jar = HTTP::Cookies->new();
# ...
my $netscape_cookie_jar = HTTP::Cookies::Netscape->new();
my $callback = sub {
    $netscape_cookie_jar->set_cookie(@_);
};
$cookie_jar->scan( $callback );
$netscape_cookie_jar->save("mycookies.txt");
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Your approach looks to me to be the safest way for the particular case of cookie-transformation.

Assuming $jar is your original cookie jar, you can just do:

require HTTP::Cookies::Netscape;
$jar->HTTP::Cookies::Netscape::save("mycookies.txt");
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No need to rebless. It could cause problems if save calls helper methods on $self which aren't defined in the parent class. But reblessing also could cause problems, so shruggles.

Hmmm, you are doing this: HTTP::Cookies::Netscape::save($jar, "mycookies.txt"); which as you say has the problem if internally save() calls any of the "overwritten" methods (quoted because they are not overwritten at all). In this case these are just save() and load(). And I see one more special-but-common case: if it recurses save(). Also calling a logger helper method would be printing parent class signature.

I am reblessing from a parent class and not just any class, so all state variables and internal methods that may be needed by the child will be inherited. That minimizes the risks. But are there still any?

I see one, if subclass has its own state variables which are set/changed during operations prior to save() for example if it implements its own set_cookie() or new(). That bug will hurt! But it will not affect subclass-specific constants and such, these will assume their initial value upon reblessing.

The use-case I had in mind is to use this "method" in order to transform from one cookie format to another using all those subclasses under HTTP::Cookies but I will have to examine each of these for above problems. I am not sure if other OO-based languages avoid this problem. I don't think Java does that or indeed can do that. It's up to the programmer to adhere to the "princaiples" (sic).

bw, bliako

Actually, these behave differently:

Some::Class::method( $obj, @args );
$obj->Some::Class::method( @args );
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If Some::Class doesn't contain a sub called method, the former will fail, but the latter will walk Some::Class's @ISA.

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You might could use Storable (or whatever your favourite deep clone module is) to get a full copy of your source instance and then instead re-bless that copy into the Netscape class (and then you could toss that clone when done). That might ensure that you're (probably) completely divested from your other instance.