As Qasic said, you have to be running a web server in order to run UBB. ActivePerl is only half of the requisites.
IIS (my server of choice) isn't included on Win98, to my knowledge. So you'll have to get another one, preferably
Apache and make sure that's set up correctly, and also make sure it's set up for Perl and stuff. Then install UBB into your webserver directory. It ought to work then, though I don't know much about setting up Apache.
---Skorpion