As you've already mentioned, support for character and byte strings is the most important new feature. In connection with localization, there's the Encode module which allows for character set conversions;
dollar sign reordering in printf;
Locale::Maketext for localized messages;
and I18N::Langinfo to query locale data.
Another feature is restricted hashes which will replace pseudohashes as the underlying mechanism of fields from perl 5.9: see Hash::Util.
Then there's safe signals, of which you've probably heard of.
Source filtering, see Filter::Util::Call.
Other smaller new features are brackets in prototypes (see perlsub),
the new ${^TAINT} variable,
newly overridable and tieable builtins,
the CLONE function,
improved attributes and lvalue functions,
loads of new core modules (such as List::Util or Tie::File etc),
better bignums,
better perlio layers.
You may want to read the short Highlights In 5.8.0 section of perl58delta.
Update 2006 oct 6: corrected spelling -- thanks to ysth.
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