prevent warnings about vstrings in use on perl 5.8.0, 5.10.0

e.g. as seen in http://www.cpantesters.org/cpan/report/73614e68-a5bc-11e3-9e9a-956311bde73e
Merge pull request #3 from karenetheridge/patch-1

prevent warnings about vstrings in use on perl 5.8.0, 5.10.0