How To enable perl5.10 by default with MacPorts

$ port uninstall perl5.8
--->  Unable to uninstall perl5.8 5.8.9_3, the following ports depend on it:
--->    perl5

So, you should type:

$ port -f uninstall perl5.8 
--->  Unable to uninstall perl5.8 5.8.9_3, the following ports depend on it:
--->    perl5
Warning: Uninstall forced.  Proceeding despite dependencies.
--->  Deactivating perl5.8 @5.8.9_3
--->  Uninstalling perl5.8 @5.8.9_3

and finally:

$ port install perl5 +perl5_10
...

which gives:

$ perl -v | grep built
This is perl, v5.10.1 (*) built for darwin-multi-2level