If you don't want to install anything on the server, you'll need either FreeTDS or the Sybase libraries.

FreeTDS is not mature enough for heavy duty production use, but for light stuff it works. Please check their site for more info. Compile that, and you'll be able to use DBD::Sybase normally. Notice that, for SQL Server 2000, I believe you'll have to compile/use FreeTDS with version 4.2 of the protocol.

The other option (Linux-only) is the Sybase libraries. I can't remember the exact location of those, but I'm sure Google will help you... Those are statical libraries (no source code provided, so watch for the system configuration requirements), and DBD::Sybase will work on top of those too.