Fixed bashrc that perlbrew init generates

zsh could not find the command "command perlbrew" as it was looking for a
binary named "command perlbrew", complete with spaces. This commit pops
and eval in the subshells so that this is no longer an issue.

It also standardizes the use of eval and subshells where necessary.
Merge pull request #288 from worr/fix_bashrc

Fixed bashrc that perlbrew init generates