cpanm's Proxy Trouble Can Depend on Its Downloader
cpanm can use different download backends. Inspect the selected path and its environment rather than assuming the interactive CPAN client's settings control it.
cpanm and the interactive CPAN client are not one configuration interface. A proxy saved for one is not proof that the other will read it.
cpanm documents several possible download paths, including LWP, curl, and Wget, depending on availability and options. Its proxy guidance starts with an appropriate proxy environment variable. The actual backend can still affect supported authentication and TLS behavior.
Identify the downloader used by the failing invocation and the environment visible to it. On Windows, a terminal opened before a persistent environment change can retain the previous value.
Use a complete endpoint in the format the selected client expects. For public CPAN downloads where the selected backend supports an HTTP intermediary and public routes are permitted, the HTTP proxy list offers candidates. A bare hostname, a PAC script URL, and an HTTP proxy URI are not interchangeable. Confirm the proxy account spelling before changing unrelated network settings.
If only one backend fails, keep that difference in the report. Do not conclude that every Perl network request is broken, or that adding an arbitrary collection of HTTP_proxy_* variables will configure every library. Use the options documented by the actual downloader and preserve certificate verification.