One of the recurring problems we face in #gentoo is end users coming
to us with confusing problems, and their problems are exacerbated
because their default workflow ended up with them unmasking some **
version of perl.

There is already a bug for this behaviour [1], and comments say that
portage doing this is "a bug", but the situations which it occurs in
are hard to diagnose what the "real problem" is.

Much of the time, what has occurred is there was some other problem,
and portage bodged its way around the real problem by choosing a
solution that should be considered unacceptable, instead of presenting
the real problem.

Some of the time, the cause is as simple as a single package being
installed that isn't in the @world dependency graph any more, which is
tripping up portage slot-rebuild behaviour.

In practice, what this currently means is that stable users end up
installing *developmental/experimental* packages that exist only for
experts and gentoo maintainers, and this is an unacceptable resolution.

If this behaviour was being triggered by anything other than portage's
dependency resolver failing, it would be considered a serious QA
violation.

Its understood that portage maintainers want to "fix" this behaviour so
the problem doesn't occur, but until that can be done, the present
default behaviour is actively harmful, and I suggest it be disabled by
default until it can be guaranteed to give the right results.

1: https://bugs.gentoo.org/658648