I don't, that's why I only use it when I'm doing debugging/verification. I usually just comment out any such quick checks if not outright delete them in the end. It's mostly just a convenience for me to make sure data structures are the same before/after some code I change.
In all honesty, I probably would have skipped learning about it had I known it was "experimental", but I went ahead and read through the chapter of Learning Perl that covered Smart Match and Given-When (also experimental) without knowing that, and only found out when I went to do the exercises at the end. I've never had a reason occur to me as to where I would use given-when, but I have found Smart Match occasionally useful for quick and dirty (i.e. lazy one line) debugging. :-)
Just another Perl hooker - My clients appreciate that I keep my code clean but my comments dirty.