It's taken from $ENV{SERVER_ADMIN} which is provided by your web server. In Apache, the ServerAdmin directive is used to set this.

If you don't have access to your web server's configuration files, or if your web server doesn't provide this environment variable, add
BEGIN { $ENV{SERVER_ADMIN} = 'webmaster@domain.com'; }
before
use CGI::Carp qw(fatalsToBrowser);.