As I know, Mojolicious does not support this feature, but you can use store and retrive functions of the Storable module this way:
use Mojo::Base -strict;
# uncomment to be sure
#BEGIN { $ENV{MOJO_USERAGENT_DEBUG} = 1; }
use Mojo::UserAgent;
use Storable;
my $cookie_path = 'my.cookie';
my $req_str = 'http://www.google.com/';
SAVE: {
my $ua1 = Mojo::UserAgent->new;
$ua1->get($req_str);
store $ua1->cookie_jar, $cookie_path;
}
my $ua2 = Mojo::UserAgent->new;
# load
$ua2->cookie_jar(retrieve($cookie_path));
# make a request with loaded cookie
$ua2->get($req_str);