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{
  package Backup::General;
...
  sub backup_dir {...}
  ...
}

use Config::Tiny;
use Module::Runtime qw(use_package);
...
my $config = Config::Tiny->read("$dir/backup.ini");
my $class  = $config->{_}{backup_class} || 'Backup::General';
use_package($class)->backup_dir($dir);