The problem is that you are decoding STDIN (via use open), and STDIN is used to transfer something that isn't text. The solution is to add 

binmode(STDIN);

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The problem in detail

CGI (the protocol) uses STDIN to pass on the document sent in POST requests. In this case, it's an application/x-www-form-urlencoded document.

Then, CGI (the module) parses that document and decodes the extracted text components.

Encode (used by CGI) detects (correctly guesses) that something's wrong and fails with 

Cannot decode string with wide characters

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Don't decode non-text. Other processing (decoding of "%" escapes) needs to be done before you have the text that needs to be decoded.

In reply to Re: Confusing UTF-8 bug in CGI-script by ikegami
in thread Confusing UTF-8 bug in CGI-script by wanradt

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