But what if the long task involves a single op code, like a sort, or a regex search with complex requirements on a large volume of data.

Or if it involves a blocking call to an external source of data, like the filesystem, network, or a DB?

Or a call to a cpan module that doesn't have any convenient callback mechanism--like any of the graphics processors, or many of the XML processors, or maths or statistics routines that need to process your large lump of data in one go?

Or a call to the OS that takes as long as it takes and provides no mechanism for smaller granularity?