Depending on how you're building that hash, you could increment your true value on a per application basis like this:

%apps = (
  gcc-3.3 => 3,
  gcc-3.3.1 => 2,
  gcc-3.3.2 => 1,
  gzip-1.2.4a => 1,
  make-3.79.1 => 1,
);
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I expect you will be querying your installed apps much more than you will be rebuilding that list. Using this method you could ++ existing keys /^gcc-/ then add your new entry with a value of 1. Then when you want the most recent you just specify => 1 or maybe even the previous version with => 2. Added benefit of not needing to know the actual release number of the previous version and your undef test will continue to work.