There is an ongoing outage on the primary CPAN mirror. It is possible to work around the issue by using MetaCPAN as a mirror.
To install Astro::Coord::ECI, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Astro::Coord::ECI
CPAN shell
perl -MCPAN -e shell install Astro::Coord::ECI
For more information on module installation, please visit the detailed CPAN module installation guide.