Inside of perldoc perlwarn there is a special category for experimental::private_use. What does private_use this warn on?
It was added by this commit, which explains the following:
I am starting to write a Unicode::Private_Use module which will allow one to specify the Unicode properties of private use code points, thus making them actually useful. This commit adds a hook to regcomp.c to accommodate this module. The changes are pretty minimal. This way we don't have to wait another release cycle to get it out there.
I don't want to document this interface, until it's proven.
Currently, the feature is used in the function that parses the interior of \p{} and \P{}.