in reply to Experimental warnings with given/when

You raise three points. First, the warnings aren't being disabled via 'use experimental "switch";' because you need to enable warnings with 'use warnings' before disabling selected warnings.

Second, I don't know why the warnings aren't going to where ever STDERR is being redirected to.

Third, smartmatch and switch were made retropectively experimental because it was widely agreed that they were broken in concept in many ways, but a consensus has never been reached on how to fix them. But the expectation is that at some point their behaviour may change, possibly radically, or they may be removed altogether.