iPhone 5 camera lens is scratched! Can it be fixed under warranty?

Only if it came that way. If it was scratched due to your use, that's not covered under warranty and you'll have to pay for service. If you have AppleCare+ coverage, the cost will low; in the US it's $49. If you do not have AppleCare+ on your iPhone, see the Service Pricing section here:

http://www.apple.com/support/iphone/service/faq/

There are independent iPhone repair services that may be able to do the repair for less than Apple's out-of-warranty charge. Do a web search for "iPhone repair". Note, though, that if you elect to have the iPhone serviced by an unauthorized shop, all further warranty and support from Apple will be voided.

Regards.