Because that's how distros are organised on CPAN. If you claim to be a CPAN mirror, you need to look like a CPAN mirror.

Special mode "cpan ." is not sufficient for your needs, but it proves useful:

#!/bin/sh

cd "$(dirname "$(readlink -e "$0")")"

for archive in *.tar.gz ; do
    distro=${archive%.tar.gz}
    tar xvzf $archive
    pushd $distro
    cpan .
    popd
done
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Error checking needed.