You can set that up in your httpd configuration:

Action do-my-thing /cgi-bin/myscript.pl
AddHandler do-my-thing
[download]

As a handler, your script becomes responsible for directing further actions. For what you're doing, it may be preferable to set this up as an output filter, rather than a handler.

That's from the Apache 2.0 manual, but I think it's the same for 1.3. A good read of the manual would be in order, there are lots of possibilities and gotchas.