If you manage your Yahoo email account using your web browser, and you have 'permanently deleted' emails, then they are gone. I assume you removed them from the account's Deleted folder.

If you manage your emails through POP to your email software on your computer, eg using Outlook Express, Outlook, WLM, Thunderbird etc, then it is possible that your web account version of your email account still has those emails in it's Deleted folder. In that case, log on to the Yahoo web server as soon as possible and see if they are still there. Also, check your email software on the computer and see if those emails are still in the Trash folder.

However, be prepared for the worse. If you cannot find them, then your only other option is to contact the Yahoo email administrators and ask if they can recover them from their backup servers.