AFAIK, Coro/AnyEvent do not bring asynchronous magic to any other perl module (e.g. Net::Telnet). So, if underlying library mechanics is blocking, they'll remain blocking with Coro.

That could be determined by module dependencies: if it does not depends on AnyEvent/EV, they are probably blocking.

Actually, in reality things could be a bit more complex, e.g. Mojolicious: it does not depends on AE/EV explicitly, but in can optionally use EV. That means, that you can use AE modules together with Mojolicious, if there is EV installed.

So, you probably should determine whether Net::Telnet supports non-blocking code style (aka depends on AE/EV); and if it is not, then find with that support or write the module by yoursef :)