I have to admit that I haven't really tested prepare_cached with DBD::Sybase. I suspect that in certain situations this might still open multiple connections, primarily if DBD::Sybase doesn't realize that a particular query is "finished" before running another one.
If I have the time I'll run a few tests to see what the deal is - and maybe add an override for prepare_cached in DBD::Sybase to avoid any bad surprises.
Update: After thinking about this a little more, I want to add that with Sybase prepare_cached is really only useful for statements that include placeholders. Any other statement won't really get cached anyway, and doesn't actually get parsed/compiled/optimized until you call DBI's execute().