Ack! Please don't use that. It does NOT indicate whether something has been decoded or not. You have been misinformed.
Yes, I saw that after I posted. Got distracted mid-post and when I got back didn't revisit the node to see the new replies.
It's the UTF-8 encoding of U+201D RIGHT DOUBLE QUOTATION MARK.
is_utf8 is even misleadingly named. I'd have called it "needs_utf8". But I won't use it.

Regarding 

use open ':encoding(utf8)';

[download]
in my earlier traversal of perlunifaq I saw
Using :utf8 for input can sometimes result in security breaches, so please use :encoding(UTF-8) instead.
in the answer to What is the difference between :encoding and :utf8? Is ':encoding(utf8)' the same as ':encoding(UTF-8)'?

In reply to Re^4: HTML::Entities and Unicode quotes by tod222
in thread HTML::Entities and Unicode quotes by tod222

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