in reply to MIME::Lite error
What mail server are you using? Your ISP's? Or somebody else's? The mail message is treated as relayed mail, so Yahoo is rejecting it.
A mail server has to be configured to allow relaying from the machine that is running your Perl script, and the recipient's mail server has to be configured to accept mail from that mail server.
Another possibility is that your ISPs mail server is not configured and allows relaying from spammers, so it's on a list of mail servers that Yahoo blocks. See ORBZ for more information on this.
This has nothing to do with the From: or Sender: headers.
In Section
Seekers of Perl Wisdom