I still haven't figured out how use open is supposed to work. Hypothesis: it doesn't actually apply the IO layer to your handle. We can test that by querying the IO layers.

use open IO => ":utf8";
while (<>) {
    my @layers = PerlIO::get_layers(\*ARGV);
    say "(@layers)";
}
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If it just outputs something like (unix perlio), it obviously didn't apply the layer.

The explicit way should work, I guess:

use strict;
use warnings;
use autodie;

binmode STDOUT, ":utf8";

for my $file (@ARGV) {
    my $fh;
    if ($file eq "-") {
        $fh = \*STDIN;
        binmode $fh, ":utf8";
    } else {
        open $fh, "<:utf8", $file;
    }

    while (<$fh>) {
        s/[[:ascii:]]//g;
        print length, " ", $_, "\n";
    }
}
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