One of the Security Updates to Mac OSX 10.5 caused problems for Perl. See
What seems to have happened is that Apple replaced some Perl modules with their older versions. These need to be reinstalled from source to get cpan and other things to work. Which modules need to be replaced seems to depend on what was updated on your system and when. Generally, you can figure out which modules you need by examining the error messages you get when installing. I had to manually reinstall IO, Scalar-List-Utils, Compress-Raw-Zlib, and Compress-Zlib from the tar distributions.
The general process is illustrated by how to reinstall IO
mkdir -p /SourceCache
cd /SourceCache
curl -O http://cpan.mirror.solnet.ch/authors/id/G/GB/GBARR/IO-1.2301.tar.gz
tar xzf IO-1.2301.tar.gz
cd IO-1.2301
perl Makefile.PL
make
make install
This was copied from an Apple discussion forum. There’s nothing special about using /SourceCache as the directory for the downloaded modules. I did have some problems with cpan mirrors failing to download the needed code modules via curl. In most cases you can get these via ftp.
See: