Drop test table

declare 
  no_table exception; 
  pragma Exception_Init (no_table, -00942); 
begin 
  execute immediate 'drop table ero_tst'; 
exception 
  when no_table 
  then 
    null; 
end; 

Create test table

create table ero_tst 
as 
select level                         id 
,      cast (null as number(3,0))    mod_count 
from   dual 
connect by level <= 10 

merge that accesses the table to be manipulated

declare
  stmnt   clob;
begin
  stmnt := 'merge  /*+ WITH_PLSQL */
            into   ero_tst    tgt
            using (select id
                   ,      id * 2    mod_count
                   from   ero_tst
                  )    src
            on    (tgt.id = src.id
                  )
            when matched
            then
              update
              set    tgt.mod_count   = src.mod_count
           ';
  execute immediate stmnt;
end;
/

Show result of merge

select * 
from   ero_tst 
order by id ;

Merge statement causing mutating table

declare 
  stmnt   clob; 
begin 
  stmnt := 'merge  /*+ WITH_PLSQL */ 
            into   ero_tst    tgt 
            using ( 
                   with 
                     function mod_x_count 
                       (id_value  in  number 
                       ) 
                       return number 
                       is 
                         return_value  number; 
                       begin 
                         -- find the number of ids that are divisible by this id 
                         select count(*) 
                         into   return_value 
                         from   ero_tst 
                         where  mod(id, id_value) = 0 
                         ; 
                         return(return_value); 
                       end mod_x_count; 
                       -- 
                   select id 
                   ,      mod_x_count(id)    mod_count 
                   from   ero_tst 
                  )    src 
            on    (tgt.id = src.id 
                  ) 
            when matched 
            then 
              update 
              set    tgt.mod_count   = src.mod_count 
           '; 
  execute immediate stmnt; 
end; 

Show result of merge

select * 
from   ero_tst 
order by id ;