You might first check to see if calendar. pl is located in www. yourdomain. com/something/something/calendar/calendar. pl
If it is there, make sure the permission is set to 755
If it is not there, check to see where calendar. pl is located (calendar_admin. pl and the folder calendarscript should also be in the same folder as calendar. pl)
If www. yourdomain. com/something/something/calendar/ is not where you have calendar. pl; calendar_admin. pl; and the folder calendarscript located, you may need to edit the path located near the top in calendar. pl and calendar_admin. pl as well as the url (2 of them) located in config. txt
Or you could just reinstall calendarscript