Perl can store Unicode strings internally in Latin-1 if no character in the string has a codepoint above 255.

That's what happens here, and it's why you don't get the "wide character" warning -- none of your characters is "wider" than 255.

Note that you can still treat $str (or $_) as a character string, and print it if you set up an :encoding(UTF-8) IO layer on STDOUT: 

$ echo -e "\xC3\xA0" | perl -CS -pne 'BEGIN{binmode STDIN, ":utf8"}; $
+_= uc'

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Update: on my perl (5.14.1) it seems that $_ is always stored in UTF8 internally, but still the point applies that no codepoint is > 255 in that string, so none is "wide".

Perl 6 - second systems done right

In reply to Re: Unicode problem with some letters by moritz
in thread Unicode problem with some letters by OlegG

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