Leave the path of customized Perl in 'superenv'

Since the path of brewed Python is left in the 'superenv', we could do the same thing for Perl (especially for people who are using perlbrew).

In particular, [the workaround for users of perlbrew in 'git'](https://github.com/mxcl/homebrew/blob/master/Library/Formula/git.rb#L31) will not work unless '--env=std' is explicitly specified, which is very inconvenient.