Known problem, which should be fixed in Perl 5.8.1. From perldelta:

       For example, if you had "en_US.UTF-8" as your locale, your
       STDIN and STDOUT were automatically "UTF-8", in other
       words an implicit binmode(..., ":utf8") was made.  This
       meant that trying to print, say, chr(0xff), ended up
       printing the bytes 0xc3 0xbf.  Hardly what you had in mind
       unless you were aware of this feature of Perl 5.8.0.  The
       problem is that the vast majority of people weren't: for
       example in RedHat releases 8 and 9 the default locale set-
       ting is UTF-8, so all RedHat users got UTF-8 filehandles,
       whether they wanted it or not.  The pain was intensified
       by the Unicode implementation of Perl 5.8.0 (still) having
       nasty bugs, especially related to the use of s/// and
       tr///.  (Bugs that have been fixed in 5.8.1)

       Therefore a decision was made to backtrack the feature and
       change it from implicit silent default to explicit con-
       scious option.  The new Perl command line option "-C" and
       its counterpart environment variable PERL_UNICODE can now
       be used to control how Perl and Unicode interact at inter-
       faces like I/O and for example the command line arguments.
       See "-C" in perlrun and "PERL_UNICODE" in perlrun for more
       information.
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Abigail