in reply to
utf8::is_utf8 valid introspection?
Code that needs to use utf8::is_utf8 (apart from for debugging purposes) is, in general, likely to be buggy.
Most of the time your code shouldn't need to care what internal format perl's currently using to store strings.
Except of course for "the Unicode bug", where the state of the utf8 flag on strings effects things like regexes for chars in the range 0x80..0xff. This has been reduced in more recent perls by the addition of things like the //a match modifier.