This hash contains coderefs which are called when various perl keywords which are hard or impossible to wrap are called. The keys of this hash are named after the keyword that is being hooked, followed by two underbars and then a phase term; either "before" or "after".
Perl will throw an error if you attempt modify a key which is not documented to exist, or if you attempt to store anything other than a code reference or undef in the hash. If you wish to use an object to implement a hook you can use currying to embed the object into an anonymous code reference.
Currently there is only one keyword which can be hooked, require, but it is expected that in future releases there will be additional keywords with hook support.
The routine indicated by ${^HOOK}{require__before} is called by require before it checks %INC, looks up @INC, calls INC hooks, or compiles any code. It is called with a single argument, the filename for the item being required (package names are converted to paths). It may alter this filename to change what file is loaded. If the hook dies during execution then it will block the require from executing.
In order to make it easy to perform an action with shared state both before and after the require keyword was executed the require__before hook may return a "post-action" coderef which will in turn be executed when the require completes. This coderef will be executed regardless as to whether the require completed succesfully or threw an exception. It will be called with the filename that was required. You can check %INC to determine if the require was successful. Any other return from the require__before hook will be silently ignored.
require__before hooks are called in FIFO order, and if the hook returns a code reference those code references will be called in FILO order. In other words if A requires B requires C, then require__before will be called first for A, then B and then C, and the post-action code reference will executed first for C, then B and then finally A.
Well behaved code should ensure that when setting up a require__before hook that any prior installed hook will be called, and that their return value, if a code reference, will be called as well. See "require" in perlfunc for an example implementation.
The routine indicated by ${^HOOK}{require__after} is called by require after the require completes. It is called with a single argument, the filename for the item being required (package names are converted to paths). It is executed when the require completes, either via exception or via completion of the require statement, and you can check %INC to determine if the require was successful.
The require__after hook is called for each required file in FILO order. In other words if A requires B requires C, then require__after will be called first for C, then B and then A.