That goes from the EGR Pipe (along the firewall) to the EGR sensor via a silicone hose on your 90 5speed. Without it being connected, you will get a code (33, I think) and you will get detonation under boost with a lack of power.
It is a real pain to get to because the cowl is in the way and there is no space to get your hand in there. The broken piece is most likely dangling on the end of the silicone hose. I'd try to remove the grey sensor as well as the piece in the hose and repair the broken nipple with some epoxy, just make sure you don't glue the hole closed. Failing that, you'll have to get another sensor. Others may say to block off the entire EGR but I don't have any info on that. Good luck