Are you sure? With perl 5.20.3:

$ perl -wE 'given (1) {2}'
given is experimental at -e line 1.
Useless use of a constant (2) in void context at -e line 1.
$ perl -wE 'given (1) {2}' 2> /tmp/err
$ cat /tmp/err
given is experimental at -e line 1.
Useless use of a constant (2) in void context at -e line 1.
$
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Are you sure?

I thought I was but I set up a little test to make sure I was sure and now I am not...

use feature "switch";

# Redirect STDERR
open my $err_fh, '>', 'stderr.log';
*STDERR = $err_fh;
# Redirect STDERR
open my $out_fh, '>', 'stdout.log';
*STDOUT = $out_fh;

my $i;
given($i) {
    print "$i\n" when $i < 4;
}

# Prove STDERR is redirected
my $error = 10 / 0;
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Illegal division by zero at test.pl line 15. is the only thing that is written to STDERR and 3 gets written to STDOUT but the following still gets displayed on the command prompt:

C:\Users\ian\Perl>perl test.pl
given is experimental at test.pl line 10.
when is experimental at test.pl line 11.

C:\Users\ian\Perl>
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So I am not really sure what is going on with output streams as the warnings are on neither STDERR nor STDOUT.

This is using Perl v5.28.1

the following still gets displayed on the command prompt

Those are compile-time warnings. They are generated before your run-time massaging of STDOUT and STDERR have taken effect. In contrast, I used the calling shell to set up the redirection so it is clear there that these warnings go to STDERR.

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🦛