You would need to setup a line similar to this in the Perl script:

$userip= $ENV{REMOTE_ADDR};

You would then need to have the $userip variable formatted into the email that is sent out.  Here are links to information on using environment variables in Perl:

http://www.pageresource.com/cgirec/ptut19.htm
http://www.4webhelp.net/tutorials/cgi/env_vars.php