I don't know how to copy the whole command prompt text...

Use your mouse or redirect to a file, see Behind the GUI lives the Shell and/or Windows Command Line Interpreter|Shell|DOS Prompt|Batch Files|Scripting ( site:http://commandwindows.com paste )

Also, cpan has an option to write install logs so you just copy/paste the log file

cpanp, which I use, calls this option install_logs, and FWIW/IDKIIM, i also have verbose 999