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