My guess is also that opening the file fails.

• Is the .xlsm-file you want to open really there? Have you tried opening it from the same directory your perl-script is in (so without a path)? In my experience Excel is sometimes picky related to path and path delimiters (slash vs. backslash).
• In my scripts I always use $Excel->Workbooks->OpenXML instead of $Excel->Workbooks->Open. Maybe this works for you?