Assuming I'm starting a project from scratch, which is not dependent on any other project.
I would like to use a format to store feeds, something like XML, since XML is not the only available format of its kind, I would like to know: why should I choose one over the rest?
I will be using perl.
'Feed' is a description of a product (name, price, type, short description, up to 120 words).
We can't really answer that without knowing a lot more. Just because you're not currently dependent on any other projects, are you likely to interact with them at some point in the future? If so, what technologies do they prefer? At the BBC, we've had some projects "JSON-only", only to find out that Java developers who wanted to access our API were begging us to provide a simple XML API simply because they have so many tool built around XML. They didn't even care about namespaces, attributes, or anything else; they just wanted those angle-brackets.
As for "storing feeds", I also not sure what you mean there. You explain the data in the feed, but what are you then going to do with those feeds? Parse them? Cache and reserve them? Write them out to cuneiform tablets? :)
I sounds like what you actually want is a database and you want to persist the data there and later make it serialisable as JSON/YAML/XML or whatever your desired format is. What I'd recommend is to be able to pull the data out into a Perl data structure and then have "formatters" which know how to serialise that data structure to the desired output. That way you can serialise to, say, JSON, and later if that's not good enough, easily switch to YAML or something else. In fact, if others need your data (one-way data tends not to be useful), they can ask for JSON, YAML, XML or whatever. You have more flexibility and aren't tied into a decision that you made up front.
That being said, I don't know your system, so it's tough to say what the right thing to do is. Also, not that JSON and YAML aren't exactly interchangeable with XML. Subtle differences can and will trip you up.