The problem is binmode doesn't change the layer of the filehandle, it adds a new layer.

Try appending the following line to your test:

warn PerlIO::get_layers($handle{$file});
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And then try to count how many times the string encoding(utf-8-strict)utf8repeats. In my case, it was 922 ×

my @l = grep /utf-?8/, PerlIO::get_layers($handle{$file});
warn @l / 2;
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To fix that, clear the layers before applying new ones by starting with :raw:

        binmode $fh, ':raw:encoding(UTF-8)';
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The result becomes agreeable:

          Rate   close noclose
close   2137/s      --    -28%
noclose 2951/s     38%      --
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And the warning now shows a pretty low number.

Updated: Show how to count the repeated layers.

Update 2: On MSWin, you might need to add :crlf after :raw if needed, too.