`whatis --wildcard *`

The * will be expanded by the shell before whatis is called. If the intent is to have * be expanded to the files in the current directory, then the --wildcard switch can be dropped, but if the intent is to have the * be used in whatis's database search (which goes beyond the files in the current directory!), the * needs to be quoted, e.g. `whatis --wildcard "*"` - or use a module that can bypass the shell, such as capturex from IPC::System::Simple.