another thing I just tested is the output of

$  perl "-V:install.*"
[download]

which confirms that "installprefix" is the /software/lib/perl5/perlbrew directoy for the perl version I have currently selected.
I read that cpanm installs modules in ${HOME}/lib/perl5 if it thinks that the current user has no permissions to install system-wide but I do have permissions on the directories under my perlbrew directory and I verified that I can write to them so I'm completely lost now what the problem might be.