libdatetime-perl 2:0.6100-2 requires libdatetime-locale-perl (>= 1:0.41), i.e. 1:0.45-1~bpo50+1. However, this one breaks libdatetime-format-strptime-perl (< 1.2000), and lenny has 1.0702-2. So, the backport is uninstallable while having anything installed that depends on libdatetime-format-strptime-perl. Unless this is serious extra effort, I suggest also backporting libdatetime-format-strptime-perl 1.3000-1.