The iusr_server user is created when IIS is installed (possibly with the installation of your OS). The user account is a local account that is used by IIS to serve content to 'anonymous' users. You can explicitly set the permissions for this user in the folder properties, security tab. The IUSR_server account will be listed in the local computer's users (as opposed to a Domain). This user needs to have read/execute access (not write).

Check out this link - http://aspn.activestate.com/ASPN/docs/ActivePerl/faq/Windows/ActivePerl-Winfaq6.html#How_do_I_configure_Microsoft_IIS from the ActiveState website for more info on setting up ActiveState Perl with IIS.