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