Perl 5.11.5 comes out tomorrow and Perl 5.12 should be out soon. (Much credit goes to people such as Jesse Vincent and David Golden, to name two, for getting Perl 5 on a regular release cycle.) I've long promised to write about the Perl 5 support and deprecation policy and how that affects users.
Perl 5.10.1 was, by definition, a minor revision. Perl 5.12 is a major revision. The nominal difference is which component of the version number increases. By intent, users of 5.10 (actually 5.10.0, but often abbreviated) should be able to upgrade that installation in place to any subsequent minor release in the 5.10 family. The upgrade isn't always completely transparent, but the intent is that, modulo bugfixes, it should be.
When 5.10.0 came out, work started on a new Perl 5 release family called 5.11 (that's not entirely true, but it's sufficiently true for this explanation). This is the unstable series intended for development and testing which will become 5.12 in the next couple of months. You are welcome to download, configure, build, test, and even install 5.11, but you should be comfortable without support from p5p for upgrades and changes.
The monthly releases in the 5.11 (and soon, the 5.13) series represent points of stability and review so that the Perl 5 developers can concentrate on the quality of what will become 5.12.0.
When 5.12.0 comes out, you will notices changes from 5.10.0 in terms of new features, removed features, and upgrades to the standard library. While most code should work unmodified with 5.12.0 as it did with 5.10.0, some modules will need updates. You likely also have to recompile any modules with XS components.
In subsequent entries, I'll write more about the implications of all of this, when you should upgrade, how deprecations and changes work, and the binary compatibility policies of Perl 5.