As mentioned in What Perl 5's Version Numbers Mean, the written Perl 5 support policy must explain several guidelines and their implications.
If you've ever upgraded between major versions of Perl 5 on the same machine, you've likely noticed that you have to install new versions of modules. Various resources spread across the Internet suggest the use of CPAN autobundles, but even that's likely enough to make you curse a little bit as you babysit a CPAN shell for an hour or two to get back to where you started.
This is due to the binary compatibility guidelines to which the Perl 5 Porters adhere. While there's a strong desire to ensure that programs and modules written in the past several years will continue to work unmodified on new major versions of Perl 5, it's almost impossible to ensure that compiled XS code remains compatible across major versions of Perl 5.
Certainly the porters attempt to maintain source code compatibility of XS wherever possible, but ensuring that an XS module compiled in 2000 for Perl 5.6.0 will continue to work unchanged on Perl 5.12 in 2010 requires a great deal of foresight, plenty of tolerance for workarounds in the core, and no small amount of luck. There's a limit to what's practical to provide for how long, and the price of reinstalling extensions (and recompiling a few) is worthwhile.
While this choice may seem odd, considering the degree to which Perl 5 retains backwards compatibility, it reflects a philosophy from the earliest days of Unix: source compatibility is important, but everyone should have access to the source code to recompile as necessary.
Within a release family—Perl 5.8.0 through 5.8.9, for example—the porters attempt to maintain binary compatibility. If you installed DBI on a fresh Perl 5.8.0, it should continue to work even if you install 5.8.9 and remove 5.8.0. That's the intent.
Note that the Perl 5 configuration and installation process enforces this by default; minor releases within a major release family reuse the same module installation paths. Major releases have new installation paths. You can reconfigure Perl 5 to use paths of your choosing, but you do so at your own risk.