The CPAN search results for Unidecode give two modules that are close but unlikely to work for your situation:

• Text::Unidecode - transliterates only US-ASCII, not Japanese characters
• Lingua::ZH::PinyinConvert - transliterates ascii pinyin (like "ni hao") into chinese characters. It includes a link to http://www.romanization.com/, but that also only goes the way from Chinese to ASCII, not the other way around and not for Japanese characters