If you have a unix/linux type system simply write the script that does the processing & emails the user (e.g. using Mail::Mailer), then in the cgi script just run the second script with the argumnets required thought a system call:
e.g.

system("myProg.pl email_add arg2 arg3 etc &");
[download]

note the & to run the job in the background. Then print oput some info to the browser like "youre job has been submitted and will email the results etc..", Obviously youll have to get the user to sumbit there email address in some html-form.