Hrrm. I'm not well versed with LWP stuff. I went to that web site with a browser, typed in an English word and got back a Japanese word (in utf8) -- that's fine (the page source had nothing strange about it). I tried wget from the command line with the url string that you would post to get that same translation:

 $ wget -O /tmp/junk 'http://babelfish.altavista.com/tr?trtext=tree&lp
+=en_ja'
[download]

and I think wget gave me the same output that went to the browser -- that's fine. (But when I tried again later, it gave me a null byte where the Japanese should have been. Having overwritten the original try, I can't be sure now.)

When I run your test script, $translation ends up with a null byte. I tried printing $result to STDERR, and redirected that to a file. The file (i.e. the full web page content returned by LWP->get) had null bytes where the browser (and maybe wget) output had a Japanese character.

So I'm guessing there is something wrong with how you are making or sending the request to the server, but I can't imagine what to try next in order to figure out the problem and fix it. Good luck.