I ran your code substituting my proxy and a real website and it worked. Perhaps your proxy is running on an alternative port (mine is on 8080). In that case, you want to add a colon and your port number after the IP address of the proxy (123.123.123.123:8080, eg.)

Another possibility is that your proxy requires authentication to use. From the docs:

$agent->proxy(['http', 'ftp'] => 'http://username:password@proxy.myorg
+.com');
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