Note that ordering is important:

#!/usr/bin/perl
use warnings;
use strict;
use utf8;

use Test::More tests => 1;
use open OUT => ':encoding(UTF-8)', ':std';

is "kůň", 1, 'same';

gives the warning, while

#!/usr/bin/perl
use warnings;
use strict;
use utf8;

use open OUT => ':encoding(UTF-8)', ':std';
use Test::More tests => 1;

is "kůň", 1, 'same';

does not.

That's why I recommended Test::More::UTF8. You can place it wherever you like and there are no warnings.

Update: <code> to <pre> to fix the non-English characters.