STDERR PerlIO layers are not preserved

[kamholz]

Setting binmode STDERR, ':encoding(UTF-8)' is not always preserved when using Capture::Tiny. The docs say it tries to do this, but evidently it doesn't succeed. The following test case demonstrates the error on version 0.44, Perl 5.24.0:

use strict;
use warnings;
use utf8;
use Capture::Tiny qw(capture capture_stdout);
use Data::Dumper;
binmode STDOUT, ':encoding(UTF-8)';
binmode STDERR, ':encoding(UTF-8)';

my $output = capture_stdout { print "eng-rus-Тверитнев\neng-rus-Тверитнев\neng-rus-Тверитнев" };

foreach my $file (split /\n/, $output) {
    print Dumper([ 1, PerlIO::get_layers(STDERR) ]);
    print STDERR $file, "\n";
    print Dumper([ 2, PerlIO::get_layers(STDERR) ]);
    my ($foo) = capture { print $file };
    print Dumper([ 3, PerlIO::get_layers(STDERR) ]);
}

The resulting output shows several "Wide character in print" warnings, and that the PerlIO layers are reset:

$VAR1 = [
          1,
          'unix',
          'perlio',
          'encoding(utf-8-strict)',
          'utf8'
        ];
Wide character in print at test.pl line 13.
eng-rus-Тверитнев
$VAR1 = [
          2,
          'unix',
          'perlio',
          'encoding(utf-8-strict)',
          'utf8'
        ];
$VAR1 = [
          3,
          'unix',
          'perlio'
        ];
$VAR1 = [
          1,
          'unix',
          'perlio'
        ];
Wide character in print at test.pl line 13.
eng-rus-Тверитнев
$VAR1 = [
          2,
          'unix',
          'perlio'
        ];
$VAR1 = [
          3,
          'unix',
          'perlio'
        ];
$VAR1 = [
          1,
          'unix',
          'perlio'
        ];
Wide character in print at test.pl line 13.
eng-rus-Тверитнев
$VAR1 = [
          2,
          'unix',
          'perlio'
        ];
$VAR1 = [
          3,
          'unix',
          'perlio'
        ];

[jimav]

This bug is still here [2025] with Capture::Tiny 0.50 and perl v5.40.1
At least with capture_stdout. capture_merged does not seem to have the problem.
Here's a trivial test case (run in terminal without redirection):

#!/usr/bin/env perl
use strict; use warnings;
use open qw(:std :encoding(UTF-8));
use Capture::Tiny ':all';

warn "BEFORE \N{U+263A}";
my $out = capture_stdout{ qx/date/ };
warn "AFTER \N{U+263A}";  # wide character warning (if stderr is a tty)

[jimav]

I think I found the bug: _capture_tee calls _open_std( $stash->{new} ); to "finalize redirection" (line 370 in Tiny.pm VERSION 0.50)

The _open_std sub unconditionally opens *STDERR to a new file descriptor even if stderr is not involved in the request (such as with capture_stdout()). I think that in that case the rest of the code thinks it does not have to restore the layers, and so they are left clobbered.