Get ruby Hash#each to return a built hash
Some notes:


Avoid the pattern init + each + update. Use map or other functional methods.
In Ruby >= 2.1.0 you have Array#to_h. Use Hash[...] in older Rubies.
Use File.join


I'd write:

def get_data(dir)
  newest = get_most_recent_file_numbers(dir)
  $file_prefixes.map do |type, file_prefix|
    path = File.join(dir, file_prefix + "_" + newest[type] + ".csv")
    [type, File.readlines(path)]
  end.to_h
end