I am now using 5.10 by mistake :) I normally only use the perl that came with my OS, which is Debian. I'm using a mix of testing (Lenny) and stable (Sarge), and a few days ago some package installation dependencies replaced 5.8 with 5.10 from Lenny. I noticed this when my scripts started failing, CPAN modules which I have installed were missing. One or two things also need fixing due to incompatibilities, like POSIX's S_IFREG et al which stopped working for me.

Haven't even started using 5.10 features, because the servers which I manage are all pure Debian stable.