perlguts.pod - add some description of real vs fake AVs

pod/perlguts.pod

@@ -645,6 +645,43 @@ of normal operations. It is therefore safest, unless you are sure of the
 lifecycle of an AV, to only use these new functions close to the point
 of AV creation.
 
+=head3 Real AVs - and those that are not
+
+The standard or typical AV is sometime referred to as being a "real" AV,
+a state that can be checked for with the C<AvREAL> macro. What this state
+basically signifies is that:
+
+=over
+
+=item * Every accessible element in the array contains either a NULL
+pointer, meaning that the value is uninitialized, or a pointer to a live SV.
+
+=item * When a SV* is assigned to an array element, the reference count on
+the SV is incremented. Conversely, when the element is unset, or assigned
+a different SV*, the reference count of the expelled SV is decremented.
+
+=back
+
+One reason for an AV to be "fake" is for better performance; this typically
+implies that the array elements will not be arbitrarily accessed.
+
+Originally this was the case for the interpreter's argument stack;
+however, this gives rise to "stack-not-refcounted" bugs when SVs are
+freed during the execution of a statement but still needed later in
+that statement. Efforts are currently underway to move to a refcounted
+stack to fix this class of bugs. (The argument stack may remain "fake"
+in that there's no guarantee that arbitrary elements beyond the stack
+pointer will be valid.)
+
+A "fake" AV can be converted into a "real" AV by C<av_reify>, but only if
+the AV has the C<SVpav_REIFY> flag set, indicating that it is safe to
+convert. The C<AvREIFY()> macro must be used to check for this flag.
+Trying to store a SV* into, or delete from, an AV usually involves an
+C<AvREIFY()> check and conversion if that macro returns true.
+
+See comments in F<av.h> and the functions in F<av.c> if you really
+want to know more.
+
 =head2 Working with HVs
 
 To create an HV, you use the following routine: