Assuming you're talking perl/sendmail...
Do you mean 'from' or 'reply-to'? They can be different eg from "Systems Admin" reply to "sysadmin@mydomain.com". Different email clients may interpret these header fields differently.
Alternatively, if your using the (now) classic, but slow perl/sendmail filehandle, you could try using a socket on the loopback and relay the message through. That way you talk SMTP to the socket and call yourself whatever you like.