By having a default IDN prefix, Version 0.03 introduces an incompatible change into IDNA::Punycode, which breaks Encode::Punycode (see http://rt.cpan.org/NoAuth/Bug.html?id=14883) and
Net::IDN::Encode.
Even worse, because IDNA::Punycode a global variable for the prefix makes it impossible to use IDNA::Punycode for modules which need a different prefix (without just doing local $IDNA::Punycode::PREFIX = 'whatever', which is a bad idea unless it that is supposed to be an official interface).