--mirror-only doesn't seem to work, but -M did!

cpanm doesn't have an -M option

You should tell cpanm that then:

$ cpanm --help
Usage: cpanm [options] Module [...]

Options:
  -v,--verbose              Turns on chatty output
  -q,--quiet                Turns off the most output
  --interactive             Turns on interactive configure (required f
+or Task:: modules)
  -f,--force                force install
  -n,--notest               Do not run unit tests
  --test-only               Run tests only, do not install
  -S,--sudo                 sudo to run install commands
  --installdeps             Only install dependencies
  --showdeps                Only display direct dependencies
  --reinstall               Reinstall the distribution even if you alr
+eady have the latest version installed
  --mirror                  Specify the base URL for the mirror (e.g. 
+http://cpan.cpantesters.org/)
  --mirror-only             Use the mirror's index file instead of the
+ CPAN Meta DB
  -M,--from                 Use only this mirror base URL and its inde
+x file
[download]

Edit: Sorry, that wasn't meant to be snarky.