Q:

How to assign a boolean value returned from a function to a variable in PostgreSQL 9.4? 

I have a function that checks whether a table exists on PostgreSQL or not, using the following code:
CREATE OR REPLACE FUNCTION public.sp_table_exists(p_in_table_name character varying)
  RETURNS boolean AS
$$
    DECLARE QUERY_COUNT INTEGER DEFAULT 1;
    QUERY_STRING VARCHAR(300);
    BEGIN
        QUERY_STRING := CONCAT('SELECT RELNAME FROM PG_CLASS WHERE RELNAME = ''',p_in_table_name,'''');
        EXECUTE QUERY_STRING;
        GET DIAGNOSTICS QUERY_COUNT = ROW_COUNT;
        IF QUERY_COUNT > 0  THEN
            RETURN TRUE;
        ELSE
            RETURN FALSE;
        END IF;     
    END;
$$ LANGUAGE plpgsql;

I'm trying to use the output of the above function to assign to a boolean value, but PostgreSQL doesn't allow me to do so. 
DECLARE DEBUG_ENABLED boolean DEFAULT FALSE;
DEBUG_ENABLED := PERFORM sp_table_exists('temp_table');

OR
DEBUG_ENABLED := SELECT * FROM sp_table_exists('temp_table');

Can you please help me resolve this? 

A:

Perform, to the best of my understanding, lets you execute a function without returning any value(s).   As such, it makes sense that this would return nothing. 
As far as assigning it to your variable, I think it's easier than you imagined:
DEBUG_ENABLED := sp_table_exists('temp_table');

select ... into is generally used when you have a field or value from a query that you want in a variable (which isn't your situation):
select count (*) > 0
into DEBUG_ENABLED
from information_schema.tables
where table_name = 'temp_table'

