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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>
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