Well look at the error you get:

USAGE: Win32::NetAdmin::GroupGetMembers(server, groupName, \@userArray)

This tells you that you have to pass the array as a reference, and this is not what you are doing:

Win32::NetAdmin->GroupGetMembers("mna.corp.mosaicco.com", "Web Proxy Users", @userArry);

This should be fixed by changing the line to this:

Win32::NetAdmin->GroupGetMembers("mna.corp.mosaicco.com", "Web Proxy Users", \@userArry);