avoid precedence warning

With Perl 5.20, installing a package with stow gives a warning like
this:

  Possible precedence issue with control flow operator at
  /gar/packages/stow-2.2.0/lib/perl5/site_perl/5.20.0/Stow.pm line 1736.

http://lists.gnu.org/archive/html/bug-stow/2014-06/msg00000.html

Suggested-by: Adam Sampson <ats@offog.org>