Moving from TSQL to PL/pgsql select into a variable...

/*
I'm trying to get to grips with the world of pgsql and have lots of habits
and techniques from a lifetime of SQL Server's TSQL.

Anyway - I'm trying to assign a variable with the value returned by a
select statement to be used subsequently within a pl/pgsql code block..
(in real life I want to maniupulate json objects)

To keep this absolutely simple as an example let's assign the value 'a' to
my variable from a select so :

SELECT cast('a' as varchar) AS X;

Then if I do the following:

*/

DO $$
DECLARE myvar varchar;
BEGIN
BEGIN
-- Try SELECT 'a' INTO myvar;
SELECT cast('a' as varchar) AS X INTO myvar;

EXCEPTION
WHEN NO_DATA_FOUND THEN
myvar := NULL;
END;
SELECT myvar;
END;
$$
LANGUAGE plpgsql ;

/*

ERROR: query has no destination for result data
SQL state: 42601
Hint: If you want to discard the results of a SELECT, use PERFORM instead.
Context: PL/pgSQL function inline_code_block line 13 at SQL statement

*/

DO $$
DECLARE myvar varchar;
BEGIN
BEGIN
-- SELECT cast('a' as varchar) AS X INTO myvar;
PERFORM cast('a' as varchar) AS X INTO myvar;

EXCEPTION
WHEN NO_DATA_FOUND THEN
myvar := NULL;
END;
SELECT myvar;
END;
$$
LANGUAGE plpgsql ;

/*
ERROR: query "SELECT cast('a' as varchar) AS X INTO myvar" is not a SELECT
SQL state: 42601
Context: PL/pgSQL function inline_code_block line 7 at PERFORM
*/

/*

I'm guessing that I'm using the wrong approach here and fighting pqsql as
opposed to using it, so can anyone point me in the right direction as to
what I should be doing to select and assign a value to a variable as I've
managed to confuse myself now.

Thanks
*/