While I don't know the answer I suspect it is because Switch is actually a source filter and it probably isn't generated correct perl for the mod_perl environment. In fact, I quote from the module's documentation.

BUGSThere are undoubtedly serious bugs lurking somewhere in code this funky :-) Bug reports and other feedback are most welcome.

I'd almost rather just point you towards something else besides Switch if only because you're introducing such oddball code that I couldn't help but laugh when I read your introductory sentence.