# perl -Mutf8 -e 'print "b" =~ /[A-C]/i ? "true\n" : "false\n"'
false
# perl -v

This is perl, v5.6.1 built for i386-linux

--

# perl -Mutf8 -e 'print "b" =~ /[A-C]/i ? "true\n" : "false\n"'
true
# perl -v

This is perl, v5.8.0 built for i386-openbsd