G'day Bod,

"I've been looking at them more for curiosity than any need to use them."

They became experimental in 5.18.0 - see "perl5180delta". Of immediate interest to you, in that rather lengthy page, are probably these sections: "The smartmatch family of features are now experimental"; and, "New mechanism for experimental features".

Also look at the documentation for the pragmata: feature and experimental. The "Ordering matters" section in the latter holds the answer to your problem.

Here's a cutdown version of what you're doing:

$ perl -E '
    use strict;
    use experimental "switch";
    use warnings;

    given (1) { say "Hello" }
'
given is experimental at -e line 6.
Hello
[download]

Taking the advice of the Ordering matters section, and the warning disappears:

$ perl -E '
    use strict;
    use warnings;
    use experimental "switch";

    given (1) { say "Hello" }
'
Hello
[download]