Well, you certainly cannot assume that Sendmail exists on a Windows machine (frankly, you probably shouldn't assume it exists on Unix either). Instead, you should use one of the many fine mail-sending modules on CPAN (for example Mail::Mailer and Mail::Send, or perhaps Mail::SendEasy by our own gmpassos). All of these work with SMTP. Of course, you will need to know the SMTP server address. Your ISP should be able to provide this, or else you can ask them to install the Microsoft SMTP Service on your box, in which case you can use localhost.

Update: that only addresses the email issue. For other portability issues, look at perlport.