begin-end blocks in psql

How do you do a simple begin-end statement block inside of a psql session?

For Example, I want to...

-          begin

-             declare a variable x to be an integer.  Set it = 5

-             declare s to be a varchar(8).  Set it to “foo”

-             insert into mytable (theint,thestr) values (x.s);

-          end

I can do it in a formal declaration of a procedure, and then execute the procedure.  But is there a less formal way?

Thanks

-dave