You can work around this by adding a call to binmode just after the open,

$fh->open(\$string, '<:encoding(UTF-8)');
binmode $fh;
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However, that only works if you do NOT use either of the ":utf8" or ":encoding(UTF-8)" options on binmode. I'm not that familiar with perl Unicode, but this seems vaguely bug-ish to me offhand.

Update: tested on perl v5.14.3 built for x86_64-linux-thread-multi