Dear Perl experts, thanks for all the usefull replies and different approaches! The PerlMonks website is a real good place to learn new things and why you choose a certain solution. Great.
The sorting is not only about dates. Patient_ID, Course_ID, Session_number, Session_date, Imaging_type, and a lot of measurements. The sorting question is the last part of a bigger project. "The doctor" needs the data in an Excel-friendly format :-( .
I can't wait to fine tune my code, but I'll have to wait till tomorrow.