I'm potting up a terrarium with some suculents. Can I use pure perlite as a medium for this?

Don't put succulents in a terrarium. It just doesn't end well for the plants.

Okay, what about cacti?

That's even worse. How about a fern or an orchid?

Okay thanks.

Think of cacti and succulents as desert dwelling plants or plants that enjoy sparse water. And you want to try to grow them in an environment that is moist and humid?!
Wow is all I can say, wow!

Yes, terrariums are usually for plants that require high humidity but you can plant a glass container with succulents or cacti as long as you don't place a lid or cover on top, which would cause excessive build up of humidity and moisture and ensure root rot. I'd place a layer of pebbles in the bottom for good drainage and use a cactus-succulent potting mix with a lot of extra perlite encorporated to help aerate the soil and aid in drainage. Be sure to keep the glass container away from direct sunlight or it could burn the foliage of your plants and since there are no drainage holes in an enclosed container .... be very, very careful with watering.

If you plant succulents or cactus in a closed bottom container, the roots will never have the appropriate dry time and will eventually fail. If you keep your succulents in their original pots with drain holes and use the terrarium as a giant saucer, it may work. But you would have to watch humidity very carefully.

I understand but an open air, arid loving plant to my way of thinking is going to do poorly because of the lack of good air movement. Little air movement can promote rots and other issues.