CREATE OR REPLACE procedure Fibonacci_Series(n  number)is
fs1 number := 0;
fs2 number := 1;
fs3 number;
cnt number default 0;
t number;
begin
dbms_output.put_line(fs1);
dbms_output.put_line(fs2);
while cnt!= n-2
loop
  fs3:=fs1+fs2;
  dbms_output.put_line(fs3);
  t:=fs2;
  fs2:=fs3;
  fs1:=t;
  cnt:=cnt+1;
end loop;
exception
when others then
dbms_output.put_line(sqlerrm);
end;

/*APPEL
EXECUTE Fibonacci_Series(20)
*/

CREATE OR REPLACE procedure Fibonacci_Series(n  number)is
fs1 number := 0;
fs2 number := 1;
fs3 number;
cnt number default 0;
begin
dbms_output.put_line(fs1);
dbms_output.put_line(fs2);
while cnt!= n-2
loop
  fs3:=fs1+fs2;
  dbms_output.put_line(fs3);
  fs1:=fs2;
  fs2:=fs3;
  cnt:=cnt+1;
end loop;
exception
when others then
dbms_output.put_line(sqlerrm);
end;
/
/*APPEL
EXECUTE Fibonacci_Series(20);
*/

Remove variable t while keeping same behaviour
Remove variable t while keeping same behaviour

The t variable had no use since there was no swapping between variables.
The use of a temporary variable would have been needed in an case like this: 
  a:=1;  b:=2;
  temp:=a;  a:=b;  b:=temp;
  a:=2;  b:=1;

Also, Add a "/" to the end of procedure to allow for multiple request in a go.