There's also this in the docs: 
CAVEAT: When you run $string = decode("utf8", $octets), then $string m
+ay not be equal to $octets. Though they both contain the same data, t
+he utf8 flag for $string is on unless $octets entirely consists of AS
+CII data (or EBCDIC on EBCDIC machines). See "The UTF-8 flag" below.

[download]
and the difference does matter from a performance point of view.

UTF-8 tagged values in perl are contagious - concatenation with an untagged value will result in a tagged value (all well and good). But the regex engine on unicode strings is slower than on byte strings.

Basically with this change in behaviour you can lose performance in a utf8-aware-and-correct application which has the vast majority of its inputs in ASCII, since the previously uncommon case of handling unicode strings is now the 100% case.

This isn't theoretical, I'm fighting a significant CPU cost increase, which adds up over many servers.

In reply to Re^2: Behaviour of Encode::decode_utf8 on ASCII by jbert
in thread Behaviour of Encode::decode_utf8 on ASCII by jbert

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