$is_ro= Internals::SvREADONLY($x) $refcnt= Internals::SvREFCNT($x) hv_clear_placeholders(%hash);
In theory these routines were not and are not intended to be used outside of the perl core, and are subject to change and removal at any time.
In practice people have come to depend on these over the years, despite being historically undocumented, so we will provide some level of forward compatibility for some time. Nevertheless you can assume that any routine documented here is experimental or deprecated and you should find alternatives to their use.
This function allows one to violate the abstraction of variables and get or set the refcount of a variable, and in generally is really only useful in code that is testing refcount behavior.
*NOTE* You are strongly discouraged from using this function in non-test code and especially discouraged from using the set form of this function. The results of doing so may result in segmentation faults or other undefined behavior.
You are strongly discouraged from using this function directly. It is used by various core modules, like "Hash::Util", and the "constant" pragma to implement higher-level behavior which should be used instead.
See the core implementation for the exact meaning of the readonly flag for each internal variable type.