The %^H hash provides the same scoping semantic as $^H. This makes it useful for implementation of lexically scoped pragmas. See perlpragma.
When putting items into %^H, in order to avoid conflicting with other users of the hash there is a convention regarding which keys to use. A module should use only keys that begin with the module's name (the name of its main package) and a "/" character. For example, a module Foo::Bar should use keys such as Foo::Bar/baz.
This variable was added in Perl v5.6.0.