Assuming .htaccess is already enabled on your server (it is on most servers running Apache), add the following near the very top to block this user-agent from accessing your site.

cat .htaccess
<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{HTTP_USER_AGENT} ^libwww-perl [NC]
  RewriteRule .* - [F,L]
</IfModule>