Oracle Datapump: modify fully qualified objects when cloning a schema
if you grant privileges (as you mention above), the problem may seem to go away, but your code executing in SCHEMA_B may be accessing data from tables in SCHEMA_A, unless that is what you want.

Unfortunately there are no easy answers to this problem, since remap_schema option of datapump doesn't really support what you are trying to do. However you may try following option (I have done it before),


Extract all plsql code and/or view definition (datapump can do this for you using impdp with sqlfile option),
use sed or any other tool of your choice and replace SCHEMA_A. to SCHEMA_B. where you see objects prefixed. Be careful, occasionally unwanted lines might get changed, so you will have to track that and implement workarounds.
impdp tables,indexes,constraints to SCHEMA_B
using sqlplus connect to SCHEMA_B and run the modified scripts from step 2 to load all plsql code, create views and other dependent objects etc.


It is cumbersome, but short of fixing your source schema, I don't think you have too many choices. Luckily if you are in a situation where code doesn't change that often, you can hold on to converted files and avoid these manual steps until something breaks. 

Oh btw, if your pl/sql code looks like "create or replace schema_a.package_name", bad luck, remap_schema won't support that either (until 11.2, I have not tested in 12c yet).