As others have said, local encoding matters.

But you also have to make sure that the POST request tells the server which character encoding you are using. I think the default is still ISO8859-1. For the downlink portion, the server sets a header like this for UTF8:

Content-Type: text/html; charset=utf-8
[download]

For a post request, you also have to set the encoding in the client. Depending on how the content of your post is structured, you have to adapt this header to your needs:

Content-Type: application/x-www-form-urlencoded ; charset=UTF-8
[download]

One easy way to do this is to look what "Content-Type" header LWP is currently sending, and then adapting it so it explicitly says "UTF-8" for the charset. It is also rather important to make sure that the data you send is, in fact, utf8 encoded.