You have to cd into the App-perlbrew directory. To install perlbrew the first time on the machine, do the following:

$ git clone https://some.valid.perlbrew.url/
$ cd App-perlbrew
$ perl perlbrew self-install 
$ perlbrew init
[download]

IIRC, if this is the very first time initializing perlbrew, you will need to use system perl to run the script. Once perlbrew is running, you should be able to just run:

 
$ cd App-perlbrew
$ git pull
$ perlbrew self-upgrade
$ perlbrew init
[download]

I've never had any problems, but since your perlbrew is already in git, you can go back to an older version if necessary.