Found the bug.

For starters, everything works fine if the server sends 

<form method="POST" accept-charset="iso-8859-15">

[download]

HTML::Form (used by WWW::Mechanize) processes that attribute and generates the correct form data. The bug is that WWW::Mechanize doesn't inform HTML::Form of the page's charset, leaving HTML::Form with no idea what to do when accept-charset is missing. (It defaults to using UTF-8.)

Some may not consider this a bug since the spec simple recommends the behaviour, but it's what other browsers do.

In reply to Re^2: www:mechanize mangles unicode by ikegami
in thread www:mechanize mangles unicode by red0hat

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