I need to be able to determine whether a domain is used for email or not.

You're on the right track.

In order for a domain to be valid for email you are required to have an MX record for it. The majority of MTAs will fail to send to your domain if you do not have an MX record. The other thing you can do is connect to port 25 for that domain name and check to see if an MTA answers.

$ telnet some.domain.tld 25 
Connected to some.domain.tld.
Escape character is '^]'.
220 some.domain.tld ESMTP Postfix
quit
221 2.0.0 Bye
Connection closed by foreign host.
[download]

There's a sample session for you to see.

As far as Perl modules go to help with this check out Net::DNS and especially look at the mx method for the default resolver. Then there is cpan:://Net::Telnet module which you can customize to connect to port 25.

Hope this helps...