Have you installed the Mail::Mailer module? See Installing Modules from the tutorials section of this site. Also of note is the README file (which contains installation instructions):

"These modules are very old (partially written before MIME!). If you start to write a new e-mail application, consider to use MailBox instead (http://perl.overmeer.net/mailbox/) It is a little harder to learn, but at least implements all RFCs correctly. "