Well for starters, I'd like to point out that these are just some suggestions, and aren't entirely related to what you're asking.

To make your life easier, as well as the lives of those who might have to maintain this, you should really check out some modules for the common tasks in your script.
CGI (included with any perl distribution) would be perfect for getting the parameters sent to this script, as well as printing the HTTP headers.
For sending email there are a whole slew of modules available, including Mail::Sendmail and Mail::Mailer.

I'm not sure exactly what runs slow with your script, but if a problem lies in the amount of time it takes recepients to get the email, there is a solution. Most likely what would cause this the local mail transport agent queueing the messages to be sent, and periodically sending them all out. This can be avoided by using a module (such as Mail::Sendmail) that will send the mail right away.