how viable it is to use perl
- Define a document more precisely than "order" or "letter". For example, are these PDFs, plain text, Microsoft formats, etc.
- Describe how documents are being sent, e.g. attachments to emails.
Without information about the data in question or the methods of movement, it's hard to evaluate the situation.
About mail, you'll also want to tell us what OS the system would be running on as well as the amount of traffic you expect to generate. In the meantime, you might try searching CPAN for send email.