Log::any::adapter::callback lets you specify callback subroutine to be called by log::any's logging methods (like $log->debug(), $log->error(), etc) and detection methods (like $log->is_warning(), $log->is_fatal(), etc.).

this adapter is used for customized logging, and is mostly a convenient construct to save a few lines of code. you could achieve the same effect by creating a full log::any adapter class.