BUG #13638: Exception texts from plperl has bad encoding

The following bug has been logged on the website:

Bug reference:      13638
Logged by:          Michal Leinweber
Email address:      lei@aswsyst.cz
PostgreSQL version: 9.4.4
Operating system:   Debian 8
Description:

I have UTF-8 database and using UTF-8 text in plpgsql and plperl functions.
Everything works ok, only exceptions from plperl functions have bad encoding
(maybe double encoded).

Compare output of these 2 functions:

create or replace function perl_test() returns text
language plperl as $$
  return "Český text ěščřžýáíé";
$$;

create or replace function perl_test_err() returns text
language plperl as $$
  elog(ERROR, "Česká chyba ěščřžýáíé");
$$;