It looks to me like you're using perl 5.8.x, as Perl 5.6.x behaves is different than your description in this regard. My answer will be in the same context.

Yes the output will be garbled, as perl thinks the contents of the string is ISO-Latin-1, and it will be "helpfully" converted to UTF-8 in the process.

You could just set the UTF-8 flag on the string, and leave the bytes as they are. One way is to use the private function _utf8_on() in Encode — well, it's not exactly private, but you're advised to use it very sparingly. Another way is to use pack this way:

$perl_utf8 = pack 'U0a*', $raw_utf8;
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I'd recommend to check if the UTF8 is in a "consistent state" afterwards, with utf8::valid(), for example.

p.s. I just came across this function in the docs for utf8:

utf8::decode($string)

Attempts to convert in-place the octet sequence in UTF-X to the corresponding character sequence. The UTF-8 flag is turned on only if the source string contains multiple-byte UTF-X characters. If $string is invalid as UTF-X, returns false; otherwise returns true.

I haven't tried it, but it sounds like something you could use.