Index.pl, index.cgi and index.html are files that do not exist in a default installation of IPB inside of the /admin folder.
My guess is when you try to access domain.com/admin/, your host is first trying to find file index.html which does not exist. It then tries to find index.pl which also does not exist. It then tries index.cgi which also does not exist. It succeeds when trying index.php.
The order in which files are tried are managed in the DirectoryIndex directive of the server.
Each of those “not founds” would trigger a 404 error. This is not an IPB problem. You can ignore the warning or have your host make the default file index.php and to try it before trying other variations.
The other issue is that an attacker is just trying to guess files and randomly just trying to load /admin/index.cgi or /admin/index.pl, etc. If this is the case, there is nothing you can do to block it within IPB as it's trying to attack something outside of the software.
In either case, it's a server/hosting related issue.