The perl package now uses a versioned path for compiled modules. This means that modules built for a non-matching perl version will not be loaded any more and must be rebuilt.
A pacman hook warns about affected modules during the upgrade by showing output like this:
WARNING: '/usr/lib/perl5/vendor_perl' contains data from at least 143 packages which will NOT be used by the installed perl interpreter.
-> Run the following command to get a list of affected packages: pacman -Qqo '/usr/lib/perl5/vendor_perl'
You must rebuild all affected packages against the new perl package before you can use them again. The change also affects modules installed directly via CPAN. Rebuilding will also be necessary again with future major perl updates like 5.28 and 5.30.
Please note that rebuilding was already required for major updates prior to this change, however now perl will no longer try to load the modules and then fail in strange ways.
If the build system of some software does not detect the change automatically, you can use perl -V:vendorarch in your PKGBUILD to query perl for the correct path. There is also sitearch for software that is not packaged with pacman.