I'm running Windows 98 on this machine and a quick search of the registry for my ip address shows that it is stored in a string value called "IPAddress" in the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans\xxxx branch. (xxxx will vary.) So I'd start by looking at all of the appropriate values using regedit. Then I would use Win32::TieRegistry to change those string values before rebooting. (Different version of Windows will probably have different registry keys.)

Another option would be to automate the task using Win32::GUITest or the like...