What is the datatype of the city_name column on the z_city table? You're selecting it into the variable called destination. Destination is defined as a varchar2(30), so what is city_name?
If you're using SQL*Plus, the easiest way to get the line numbers is to type "list" (or just "l" for short) in the same session after you've compiled your PL/SQL. Line 71 in your text editor may not be the same as what Oracle thinks line 71 is!
By the way, http://tahiti.oracle.com/ is a great site for looking up Oracle error messages and documentation. Oracle have really made a fine effort to make their docs more available.
