debian:/usr/local# apt-get install mysql-client-5.1
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
mysql-client-5.1: Depends: libdbi-perl but it is not installable
Depends: libdbd-mysql-perl (>= 1.2202) but it is not installable
E: Broken packages
debian:/usr/local#

This error message is probably caused by your limitation in /etc/apt/preferences

debian:/var/lib# vi /etc/apt/preferences

Package: *
Pin: release o=Debian,a=stable
Pin-Priority: 900
Package: *
Pin: release o=Debian
Pin-Priority: -1
Generally you should remove this file, you would only need to play with that file if you (manually) want to mix and match different mirrors. It's difficult to do right, and usually leads to problems. Or at leat: it leads to confusion...