How to mute that when and given are experimental in Perl?

First of all, note that smartmatching will be removed or changed in a backwards incompatible manner. This may affect your given statements.

---

To use given+when without warnings, one need the following:

# 5.10+
use feature qw( switch );
no if $] >= 5.018, warnings => qw( experimental::smartmatch );

or

# 5.18+
use feature qw( switch );
no warnings qw( experimental::smartmatch );

experimental provides a shortcut for those two statements.

use experimental qw( switch );

---

Finally, you ask how to add this to your programs without changing them (and presumably without changing Perl). That leaves monkeypatching.

I wouldn't recommend it. It's far easier to write a couple of one-liners to automatically fix up your programs that rewritting Perl's behaviour on the fly.

But if you want to go in that direction, the simplest solution is probably to write a $SIG{__WARN__} handler that filters out the undesired warnings.

$SIG{__WARN__} = sub {
   warn($_[0]) if $_[0] !~ /^(?:given|when) is experimental at /;
};

(Of course, that won't work if your program makes use of $SIG{__WARN__} already.)

To get it loaded without changing your programs or one-liners, all you have to do is place the patch in a module, and tell Perl to load the module as follows:

export PERL5OPT=-MMonkey::SilenceSwitchWarning

---

$ cat Monkey/SilenceSwitchWarning.pm
package Monkey::SilenceSwitchWarning;

use strict;
use warnings;

$SIG{__WARN__} = sub {
    warn($_[0]) if $_[0] !~ /^(?:given|when) is experimental at /;
};

1;

$ perl -e 'use v5.10; given (12) { when (12) { print "Hello World\n" }}'
given is experimental at -e line 1.
when is experimental at -e line 1.
Hello World

$ export PERL5OPT=-MMonkey::SilenceSwitchWarning

$ perl -e 'use v5.10; given (12) { when (12) { print "Hello World\n" }}'
Hello World

no warnings 'experimental';

...works in version 5.22 at least.