when the internal data is already known to be the same as how it should be written (UTF-8).

syswrite and print print the string, not its internal data. The internal storage format is not relevant here.

>perl -we"$_ = chr(0xE9); utf8::downgrade($_); print;" | perl -nE"say 
+length;"
1

>perl -we"$_ = chr(0xE9); utf8::upgrade($_); print;" | perl -nE"say le
+ngth;"
1

>perl -we"$_ = chr(1000); utf8::upgrade($_); print;" | perl -nE"say le
+ngth;"
Wide character in print at -e line 1.
2
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