While your CGI script may be in the same directory as the Excel file, the script may be running with a different directory as its current working directory. You can check this by adding:

use Cwd;
print cwd(), $/;
[download]

to your CGI script.

Additionally, your CGI script may be run as a user with less permissions on the system than you have. This is actually extremely common, especially to the directories which contain CGI scripts. This is a security measure against unauthorized people on the internet reading the code for your CGI scripts.

I would recommend running the script from the Windows commandline and seeing if you still get the same error. If you don't, then it's a problem with how the webserver is running your script. If you get the same error, then it's something we can help investigate.