Only as of 5.14 are most of the builtin operations fully aware of Unicode; too many of them default to assuming the semantics of whatever the local character set is. unicode mentions this as "The Unicode Bug". Perl 5.12 fixed some of that, but only as of Perl 5.14 can you use use feature 'unicode_strings'; to get Perl to do the right thing.
I haven't encountered huge problems in my work, but most of what I do uses nothing more exotic than Latin-1 and UTF-8, so I've been fortunate. Then again, everything I care about runs on Perl 5.14.1 (and soon Perl 5.14.2).