Retrieving this url with firefox gives me:

Your connection is not secure

The owner of www.earthquake.usgs.gov has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.

Learn more…

So it looks like LWP::Simple does https requests with certificate validation turned on by default.

update: So use something like this:

perl -MLWP::UserAgent -e 'print LWP::UserAgent->new(ssl_opts => {verif
+y_hostname => 0})->get($ARGV[0])->content' https://www.earthquake.usg
+s.gov/earthquakes/feed/v1.0/summary/2.5_week.atom
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