Q: How can list the current settings of config variables used by Perl's cpan? Among the many command line options of the cpan, which one reports the config variables currently set in cpan? I made some changes with o conf and want to see if the values are correctly has been set because I don't see their effects.

A: perldoc cpan:

-J:  Dump the configuration in the same format that CPAN.pm uses. This is
          useful for checking the configuration as well as using the dump as a
          starting point for a new, custom configuration.

You should also be able to get the same (or possibly more) information by running the CPAN shell:
 $ perl -MCPAN -e shell

 cpan shell -- CPAN exploration and modules installation (v2.16)
 Enter 'h' for help.

 cpan[1]> o conf
 $CPAN::Config options from C:\...\.cpan/CPAN/MyConfig.pm:
 ...
 applypatch         []
 auto_commit        [0]
 build_cache        [100]
 build_dir          [C:\...\.cpan\build]
 build_dir_reuse    [0]
 build_requires_install_policy [yes]
 bzip2              [C:\opt\bin\bzip2.EXE]
 ...

