Your .htaccess (or httpd.conf) is probably set to only authenticate GETs not POSTs. Fix this and your REMOTE_USER will be fixed.

Something like the following should fix you up:

  <Directory /your/dir/here>
    AuthUserFile /some/path/htpasswd
    AuthType Basic
    AuthName PasswordRequired
    <Limit GET POST>
      require valid-user
    </Limit>
  </Directory>
[download]

This is not really a perl problem but an apache problem (feature really) that GET and POST (and PUT, etc) can all be authenticated independently.

-monkfish (The Fishy Monk :)