Radar is the model of the glasses. The XL is just the type/size of the lenses. They also have Range, Path, Pitch, and Edge lenses (different shapes/sizes) as well.

This shows the lens differences: http://www.oakley.com/radar

Check with Oakley customer service. They often do lens replacement. At one of our races last year they swapped out lenses for free

If you have an Oakley store near by you can head in there and get it fixed same day.

The first poster that responded to your question is incorrect. The Radar XL or XL Blade is a separate model than the standard Radar. The XL has a larger lens as the bridge from nose piece to top of frame is a greater distance than the standard Radar...hence uses a lens specific to that frame. The standard Radar frame can except pitch,path or range lens these will not work with the XL
There should be small white or black print on the inside of one of the arms with a model code and if memory serves me should say radar XL