Not sure quite sure what you mean by permanently, but maybe these will help:

• Really Permanently: Why not go through Settings -> Control Panel -> System -> Advanced -> Environment Variables?
  
• Win32::Registry The keys you are looking for are:
  1. HKCU\Environment - User (saved between logons)
  2. HKLM\System\CurrentControlSet\Control\Session Manager\Environment - System
  3. HKCU\VolatileEnvironment - User (not saved between logons)
• Meta::Utils::Env: May work for you, check the docs.

Hope these help,

Smitz