After upgrading to 5.18.0, I too was annoyed by the dev's decision to now make use warnings now also spam "Smartmatch is experimental" messages, since I used given/when/~~ in some of my scripts.
But after more research, it seems there is a good reason for those messages. From perlbug #116913: experimental warning for given/~~ before 5.20.0:
The behavior of given/when/~~ are likely to change in perl 5.20.0: either smart match will be removed or stripped down. In light of this, users of these features should be warned.
So it seems the annoyance is intended, to motivate us to phase out the usage of those features in our scripts.
Still, I think the point would have come across more clearly if the warning would read "Smartmatch is deprecated" instead of "experimental"...