I realise the quoted text is from blead but are you saying the :utf8 IO layer in earlier perls (say 5.8.8 for example) just sets the utf-8 flag without checking the encoding? If so then I don't understand the following in 5.8.8

od -x x.data
0000000 8181 8282 8383 000a
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use strict;
use warnings;

my $fh;
open ($fh, "<:utf8", "x.data");
my $img = '';
while (<$fh>) {$img .= $_;}
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produces 1 utf8 "\x81" does not map to Unicode at invalid_utf8.pl line 8, <$fh> line 1.

but changing the io layer to :encoding(UTF8) seems to make no difference other than reporting that same error 6 times, one for each byte.