Absolutely yes. It's a permanent license, Adobe will honor activation and it will run unless you upgrade your OS to the point where it no longer supports CS5.
In the event Adobe discontinues activation, they will either provide a patch or activation free version.
...so, am I right that with CS6 (on a perpetual licence) I can deactivate and uninstall it, subscribe to Photoshop CC and then reinstall CS6 when I get fed up with the subscription? I thought I read somewhere that once you upgrade a version you can't go back. Thanks for any clarification.
I don't think you have to uninstall, just deactivate.
If you do not have disks it is a good idea to burn the download files, record any pertinent info and store in a safe place for CS5 and 6. May need them in future if you need to reinstall.