How about just overriding the HTTP::Response::content method then?

require HTTP::Response;
use Carp 'cluck';

my $old_content = \&HTTP::Response::content;
*HTTP::Response::content = sub {
    cluck "HTTP::Response::content called - do you want HTTP::Response
+::decoded_content instead?";
    goto &$old_content;
};
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You could get fancy and check whether it's HTTP::Response::decoded_content that is calling you and then suppress your warning, by using caller.