I think it works as designed.  In other words, if you concat a unicode/character string with a non-unicode/byte string, the byte string will automatically be upgraded to unicode, with the non-ASCII values being interpreted as if they were in Latin-1 encoding — i.e. the first byte \xc2 (Â in Latin-1) becomes Unicode-Â (which happens to be codepoint U+00C2) encoded as UTF-8 (i.e. the bytes \xc3\x82), the second byte \xa9 (© in Latin-1) becomes Unicode-© (codepoint U+00A9) encoded as UTF-8 (i.e. the bytes \xc2\xa9), etc...

Update: if you print a hexdump of your string $x, e.g. 

sub hexdump {
    my $s = shift;
    print join " ", unpack("(H2)*", $s), "\n";
}
# ...
hexdump($x);

[download]

you'd get 

c3 82 c2 a9 c3 82 c2 ae c3 a2 c2 84 c2 a2 c2 a9 c2 ae e2 84 a2

[download]

with the first 4 bytes showing the result (UTF-8 encoding) of the conversion I tried to describe above.

Or, fully expanded: 

_________ $r (auto-upgraded) _________    ________ $u ________
c2    a9    c2    ae    e2    84    a2    c2-a9 c2-ae e2-84-a2
|     |     |     |     |     |     |     |  |  |  |  |  |  |
c3-82 c2-a9 c3-82 c2-ae c3-a2 c2-84 c2-a2 c2-a9 c2-ae e2-84-a2
     ©          ®     â     U0084 ¢     ©     ®     (TM)

[download]

In reply to Re: Problem with join'ing utf8 and non-utf8 strings (bug?) by almut
in thread Problem with join'ing utf8 and non-utf8 strings (bug?) by rsmah

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