Forking in Windows is simulated by threads underneath. Win32::OLE is not thread safe plus the Excel automation object lives in the STA so the call must be serialized

From : Threading Support in Office

"The Office object model is not thread safe, but it is possible to work with multiple threads in an Office solution. Office applications are Component Object Model (COM) servers. COM allows clients to call COM servers on arbitrary threads. For COM servers that are not thread safe, COM provides a mechanism to serialize concurrent calls so that only one logical thread executes on the server at any time. This mechanism is known as the single-threaded apartment (STA) model. Because calls are serialized, callers might be blocked for periods of time while the server is busy or is handling other calls on a background thread. "