Only re-exec perlbrew env if a name is provided.
Only re-exec perlbrew env if a name is provided.

This issue fixes GH #261

ptolemarch/App-perlbrew,ptolemarch/App-perlbrew,dagolden/App-perlbrew,rafl/App-perlbrew,Asuza/App-perlbrew,dagolden/App-perlbrew,rafl/App-perlbrew,calid/App-perlbrew,rafl/App-perlbrew,josl/App-perlbrew,calid/App-perlbrew,josl/App-perlbrew,josl/App-perlbrew,Asuza/App-perlbrew,Asuza/App-perlbrew,ptolemarch/App-perlbrew