Note that utf8::valid is an internal method, and shouldn't be needed or useful in production code.

\x{c3a9} is not a valid unicode codepoint; I think you meant \xc3\xa9. But even that won't match, because perl still treats the string as a sequence of characters, the third of which is the unicode code point 00E9. If you want to create string where each character is a byte of a utf8-encoded string, you want to be using Encode, not the utf8 functions:

$string = encode("utf8", $string);
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This should do exactly the same thing whether you've done utf8::upgrade($string) or not.