I don't have any direct suggestions as to what you're doing wrong, but you could try the Dependency Walker (depends.exe¹), a nifty application to debug problems with shared library loading. Highly recommended, BTW (hehe, me (a hardcore Unix gal) recommending a Windows tool, that does mean something... ;)

Either load your DLL associated with the DBD::Sybase extension into the tool, or - preferably - use its "profiling" feature, i.e. watch a running application (perl, in this case) to see what dynamic libs it loads (or tries to load). For this, open perl.exe and run it (F7), with your script supplied as program arguments. Then inspect the module dependency tree, version numbers, etc... (you probably want to press F9 to see the absolute paths). Dependency walker comes with a reasonably good online help, for example explaining the different types of dependencies, like implicit, explicit, forward, delay-load... I'd suggest you read it :)

As you're using Win XP, it might be a problem with incorrectly set up or missing side-by-side assemblies — a feature introduced with XP, which allows applications to specify specific versions and/or locations of files they depend on, e.g. shared libs. This is closely related to manifest files, in other words, messing with those might be a way to work around (or at least investigate) problems of this kind. Unfortunately, this whole topic is rather complex, so I won't even attempt to describe any details here. If you're interested, you might want to follow the links from the (more or less arbitrarily chosen) pages I linked to.

Troubleshooting C/C++ Isolated Applications and Side-by-side Assemblies might be another good page to start.

Hope this helps,
Almut

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¹  depends.exe should come with the respective compiler suite (if you opt to install the Platform SDK), typically in a directory ...\tools\bin. In case you really can't get hold of it, you could also try dumpbin /DEPENDENTS