Improve global vars for perlcc (DMUEY)

    - simplify scalar-via-blob syntax
    - account for both byte strings and unicode strings
    - add SMALL COMMERCIAL AT to list of possible @ signs

Prompted by https://code.google.com/p/perl-compiler/issues/detail?id=143

All tests pass when run under 5.6.2, 5.8.8, 5.16.0, and 5.18.1