Fixes test for perlbrew arguments in fish shell

Previously the error "test: unexpected argument at index 2: 'perl-5.18.2'" was reported when perlbrew was called with `perlbrew switch perl-5.18.2`. By interpolating the arguments into a string test receives a single argument rather than an array of arguments.
Merge pull request #378 from tgt/patch-2

Fixes test for perlbrew arguments in fish shell