The asterisk wildcard character seems not to be supported in the domain of a whitelisted cookie in the Mac version, while it is in the Windows version.
In the Windows version, while it isn't possible to add a domain containing a wildcard to the list of cookies to be kept (the right side of Options > Cookies) using the GUI, such a pattern can be added to the list in the registry and it will subsequently appear in the GUI. CCleaner itself comes preconfigured with a "*.piriform.com" whitelist entry.
In the Mac version (at least as of 1.07.236), the situation doesn’t seem to be analogous. The app doesn’t come with any “*.piriform.com” whitelist entry, and any such entry added to the CookiesToKeep dictionary in ~/Library/Preferences/com.piriform.ccleaner.plist manually doesn’t appear in the list in the GUI. I haven’t tested whether such an exception still operates, despite not appearing in the GUI, but I doubt it.
I'm curious why this difference exists between the Windows and Mac versions. Is it, for example, due to the API used to read the plist file? (I wouldn't think Apple would tolerate such a restriction since plists are used everywhere in OS X.)
Any Piriformer who sees this: I’d much appreciate whatever brief explanation you might give.