It will always require 5 Perl interpreters to handle 5 requests at once. However, mod_perl is much more efficient than the CGI approach being used by IIS because it doesn't need to fork a process and the interpreter is already loaded. The code will stay in memory once it is compiled the first time as well.

If you'd prefer to stick with IIS, look at ActiveState's PerlEx product, which offers functionality similar to mod_perl for IIS.