Well, yes and no. No, you can't assume that the user is using sendmail, but yes, you should assume that he'll be running some kind of mailserver on the local machine, at least if he know what he is doing. Any other configuration will effectively break all kinds of redundancy and failrecovery built into SMTP.

A Perl script is non-persistent, like any other MUA (Mail User Agent). In other words, if the MUA can't deliver the mail it must fail, even though it might be possible to recover, according to SMTP rules. Many MUA won't even handle MX records, and will fail if the first mailserver in the MX records won't answer, or even worse, will fail if there are just MX records and no A/CNAME records, which is a rather common situation. This problem could be fixed, although there would be problems with the script blocking until the mail is actually delivered, which may take several minutes. Not a situation you wan't in a CGI script. And it still won't be able to handle queuing.

In my opinion, it's bad design to have a MUA deliver anywhere else than to a mailserver in the local machine. In worst case deliver to a local mailserver that can act as a relay, for example to handle firewall issues. It's extremely bad karma to deliver directly to remote mailserver from a MUA. It will only cause people to call you at 3AM claiming that mail doesn't work.