use WWW::Mechanize;
use HTTP::Cookies;

my $mech = WWW::Mechanize->new();

my $cookie_jar = HTTP::Cookies->new(
                                    autosave => 1,
                                    file => "/your/cookies.txt"
                                   );
# $cookie_jar->clear();

$mech->cookie_jar($cookie_jar);
$mech->agent_alias('Windows IE 6');

$mech->get("http://www.amazon.com");

my $self_enclosed_callback = sub {
    my ( $version, $key, $val, $path, $domain, $port, $path_spec,
         $secure, $expires, $discard, $extra ) = @_;

    # Remove the currently iterating cookie from the jar.
    # NB: this might be dangerous! Seems to work though.
    $cookie_jar->clear( $domain, $path, $key );

    # Now change domain, just for example.
    $domain =~ s/\.com\z/.org/;

    $cookie_jar->set_cookie( $version, $key, $val, $path, $domain,
                             $port, $path_spec, $secure, $expires,
                             $discard, $extra );
};

# Before our callback.
print $cookie_jar->as_string, "\n";

$cookie_jar->scan( $self_enclosed_callback );

# After our callback.
print $cookie_jar->as_string, "\n";