The singleton class idea is probably best, but here is another option: your main script could create your one instance and assign it to a global variable, which the other modules could access at will. Something like the following then would be at the top of each file:

use Login; use Config;
our ($login, $config) = (Login::new(), Config::new());
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This is in some ways less elegant than the singleton, but it leaves you the option that some modules can create their own instances if desired, but others can use the main instance.