You'd need to use Win32::API or Inline C to do this. See here where is says:

To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message with lParam set to the string "Environment". This allows applications, such as the shell, to pick up your updates.

No guarentees that a pre-existing shell instance would see your changes though. You might be able to inject code into the parent process to do it, but that's nasty and awkward.