Works great in C code where read accesses to variables doesn't change anything.

In perl, where a read access can cause change: like using a string scalar (PV) in a numeric context; or an integer or real scalar (IV or NV) in a string context; or taking a reference to any type of variable; or an other operation that causes the SVs contents to change as a result of a Perl level read access; and the 4k page that SV lives in, has to be copied.

Still beneficial, but not always as beneficial as it at first might appear.