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 Tk::XMLViewer, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Tk::XMLViewer
CPAN shell
perl -MCPAN -e shell install Tk::XMLViewer
For more information on module installation, please visit the detailed CPAN module installation guide.