Fixed bug with perlbrew env where PERL5LIB would be unset

If PERL5LIB was naturally unset, perlbrew env wouldn't find the perlbrew
supplied PERL5LIB. I've fixed this, assuming that PERL5LIB is always
in the last position of PERL5LIB (which your code seemed to assume as well).
Merge pull request #287 from worr/fix_perl5lib_env

Fixed bug with perlbrew env where PERL5LIB would be unset