** placed in my CS-directory. **
Placed in your CS-directory
how exactly??

What permissions are the /calendarscript/ directory set to. It shouldn't need to be any more than 666 but setting it to 600 should prevent any scrips or files in it from being executed or even read from a browser but shouldn't affect the script's ability to use them.
If you're on a Unix based host, it should be possible to curtail the running of
all PHP scripts in a particular directory using an .htaccess file.
** running CS 3.21 **
You're absolutely certain? You actually looked at the .pl files for their version number listed at the top? Is so, it may not be responsible for the file upload. Someone will need to investigate how those file(s) are being uploaded.
JMO
Dan O.