Although it isn't incorrect to hand craft e-mail (even with attachments), it's much easier if you let someone else worry about the niggling details. I recommend you use Email::MIME to do the construction, print it as a string, then send that to your invocation to Sendmail.

You specifically want to send an attachment that's an Excel compatible file. The very simplest is to make a CSV file and set the MIME type to text/csv. Excel will handle that gracefully when the recipient gets the e-mail using their favorite e-mail client. If you want to go more advanced, then Spreadsheet::WriteExcel will give you the ability to create an actual binary Excel file that can be attached as the MIME type application/vnd.ms-excel.