To be more specific - there is no such thing as a sub-module, save in convention. Modules ABC and ABC::DEF have no relationship other than the following:

1. The files live in relatively similiar directory structures.
2. The programmer has a mental picture that they're related

Perl doesn't make any package relationships for you.

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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.