We don't currently mention PERL_CORE in perlguts.pod.
I'm wondering whether we should mention it, purely in the negative context of
stopping people cargo-culting it: i.e. just because they copied some XS from
a module that used it, doesn't mean that they should use it too.
Something along the lines of:
"Don't use this. It exposes the perl innards, which change between
releases and thus your code will break with every new release"
Also it means that someone who complains that their XS has broken can be
pointed at the docs.
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My get-up-and-go just got up and went.
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