It looks like the behaviour changed (i.e. was fixed) between 5.6 and 5.8:

    $ perl5.6.0 -Mutf8 -e 'print "b" =~ /[A-C]/i ? "true\n" : "false\n
+"'
    false

    $ perl5.8.0 -Mutf8 -e 'print "b" =~ /[A-C]/i ? "true\n" : "false\n
+"'
    true
[download]

--
John.