More context would be helpful. Where is your input string coming from, and what do you mean by "real value"?

Text::Unidecode only aims to provide a rough ASCII transliteration of the underlying characters, and it is self-admittedly quite bad at Thai. Is this really what you want?

Update: also, if you don't know about it already, be sure to check out Encode, the core Perl module that deals with character encodings.