DECLARE
	begin
		INSERT INTO T1 VALUES(3);
	end;

SQL> @path/first.sql
6  

DECLARE
	begin
		INSERT INTO T1 VALUES(3);
	end;
/

SQL> @path/first.sql

PL/SQL procedure successfully completed.

 select * from t2;

         X          Y          Z
---------- ---------- ----------
         1          2          3


INSERT INTO T2 VALUES(4,5,6);
/

SQL>  @path/first.sql

PL/SQL procedure successfully completed.


1 row created.


1 row created.

SQL> select * from t2;

         X          Y          Z
---------- ---------- ----------
         1          2          3
         4          5          6
         4          5          6

The semi-colon (;) loads it into the buffer, the slash (/) executes it.