I use a CPL filter when appropriate, which pretty much means anytime I shoot outside.
As a long-time photographer I have never put UV or clear filters on my expensive lenses for protection of the front lens objective.
I do put on a skylight filter on when shooting at altitudes approaching and above 3000 meters (10,000 feet). If I am shooting a landscape on a hazy morning, I'll shoot a couple of frames with a skylight filter on in case it helps with the haze.
Shooting film, filters play a much larger role in making the image than they do with digital image sensors. Shooting film, using filters is pre-processing an image and with film UV filters play an image enhancing role. UV filters have no positive effects on digital images. At best they have no effect, at worse they degrade image quality, and if they shatter can damage the lens objective glass they were intended to 'protect'.
Instead, for protection I now rely on a good lens hood, and as always, good camera handeling technique, situational awareness, and time proven non-scratch/abrasive cleaning methods to ensure unmarred lens objectives and coatings.
Inside nothing, except a lens hood, is needed on the front of a lens unless you want to use a special effects filter.