This was found here: http://www.unix.com/shell-programming-scripting/12995-perl-emailing-multiple-files-attachment.html This was really hard to find (google-fu may have been weak) so thought I should spread it around.. Make your message Type "multipart/mixed" and for the attachment:

...
        $msg->attach(
               'Type' => 'application/octet-stream',
               'Encoding' => 'base64',
               'Path' => "<some Excel file here>"
               );
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