dev-lang/perl: Make new OLDVERSEN strategy for NEXTVER

As per discussion on #gentoo-perl, this is slated as a strategy for
5.26, as it means people who upgrade to 5.26 retain existing installed
modules in their @inc

This serves as additional defence against portage build-order issues,
which are going to become especially pertinent given we're going to
have a substantial number of packages likely to fail configure or test.

So its important that people who can't rebuild packages can still have
existing versions continue to work on the newer perl where possible.

Package-Manager: Portage-2.3.5, Repoman-2.3.2

dev-lang/perl/perl-5.26.9999.ebuild

@@ -10,13 +10,12 @@ CROSS_VER=1.1.4
 
 DIST_AUTHOR=XSAWYERX
 
-# NB: BIN_ are perls that are XS-Compatible
+# Greatest first, don't include yourself
+# Devel point-releases are not ABI-intercompatible, but stable point releases are
+# BIN_OLDVERSEN is contains only C-ABI-intercompatible versions
+PERL_BIN_OLDVERSEN=""
+PERL_OLDVERSEN="5.25.12 5.25.11 5.24.2 5.24.1 5.24.0 5.22.3 5.22.2 5.22.1 5.22.0"
 if [[ "${PV##*.}" == "9999" ]]; then
-	# Include more versions for blead releases
-	# for circular reasons
-	# Greatest first, don't include yourself
-	PERL_BIN_OLDVERSEN=""
-	PERL_OLDVERSEN="5.25.12 5.25.11 5.24.2 5.24.1 5.24.0 5.22.3 5.22.2 5.22.1 5.22.0"
 	DIST_VERSION=5.26.0-RC1
 	SHORT_PV="${DIST_VERSION%.*}"
 	# Devel Releases are not ABI-intercompatible
@@ -25,9 +24,6 @@ if [[ "${PV##*.}" == "9999" ]]; then
 	PATCH_BASE="perl-5.25.11-patches-${PATCH_VER}"
 	MY_PV="${DIST_VERSION%_rc*}"
 else
-	PERL_BIN_OLDVERSEN=""
-	# Compat reasons
-	PERL_OLDVERSEN="5.25.12 5.25.11"
 	# First 2 digits only
 	SHORT_PV="${PV%.*}"
 	SUBSLOT="${SHORT_PV}"