$x = "\xFC";        # ISO-Latin-1: 1 byte
$x .= pack 'U0';    # append a zero length UTF-8 string
                    # which will turn the whole thing into UTF-8!
# hex dump:
local($\, $,) = ("\n", " ");
print map { sprintf "%02X", $_ } unpack 'C*', $x;

##</code><code>##

  C3 BC

##</code><code>##

$l1 = pack 'C0a*', $s;    # copies the bytes out of $s and marks the result as non-UTF-8

$u8 = pack 'U0a*', $s;    # copies the bytes out of $s and marks the result as UTF-8 -- even if the bytes don't form proper UTF-8 strings!