CreateProcess allows this via the lpStartupInfo parameter. Unfortunately, Win32::Process does not give access to that structure.

It would probably be a relatively trivial to add a second constructor to the class definition that underlies that module, to take a hash to fill in the structures values, but that may be further than you wish to go.

You might also be able to use Win32::GuiTest to reposition the window once it has been created. But that process would be visible to the user, and given you won't know the original position, would require a bit of thought to make work reliably. Modifying Win32::Process is probably easier.