Umm, define "core". The perldoc to Switch states "There are undoubtedly serious bugs lurking somewhere in code this funky" and then goes on at some lengths about the limitations of source filters. TheDamian (the original author) categorizes Switch as a module "...you shouldn't use in production...", because it's an experiment. Given that, it'd take some pretty serious convincing for me to use the module for anything serious(especially since the functionality it implements is otherwise available).
Update: itub defined "core" for me, thanks :-). I missed it because Switch.pm isn't listed at the perl.5.8.7 page at search.cpan.org, though the download does contain it. Curious indeed.
Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. -- Brian W. Kernighan