Sometimes when you install a new perl version, you want to put all the modules you were using on a previous version onto your new perl install.
You can easily get the list of modules in the current perl installation with
perlbrew use perl-5.8.8@foo-project perlbrew list-modules
That simply prints out module names that you can use latter.
This one-liner can install everything in that output to a diferrent perl installation:
perlbrew list-modules | perlbrew exec --with perl-5.16.2@foo-project cpanm
exec command is particularly helpful to run any program as if it is in the
environment of the specified perl installations. Understand it more by running
perlbrew help exec.