I've only used ClearCase and Subversion as well. And ClearCase was my first version control system I'd ever used, so I didn't have a very good basis for comparison.
But I do have to say that I had decent results with using ClearCase. The 'find' and 'describe' commands were complex enough to write Perl scripts as a wrapper, which led to additional scripts that would do things like listing which files changed between given labels. In short, I found that ClearCase was fairly easy to deal with on the command line, and haven't yet gotten up to speed on the same level with Subversion.
But what I'm really curious about is why a given version control system is better than another. Hmm. Maybe a Meditation-worthy question (but pretty OT to Perl itself).