In short: It depends!

In long: ...

In order to change an arbitrary given config format you'd use a bunch of regex.

If a fixed format is known beforehand people would prefer templates (plenty on cpan) to create it.

Others define their own DSL (set of functions representing cfg actions)

If the format language could also be dynamically expanded in future versions, one would also need rules for a parser and validator.

This whole field is like manipulating XML, here might be dragons.

update

It's actually even more complex than XML because the example you are showing looks a lot like bash and inserts $variables.

I.o.W. it's already a DSL.

You are asking for a module which manipulates any other config language in a generic way.

Holy grail...