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(at)aswsyst(dot)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 $$
elog(ERROR, "Česká chyba ěščřžýáíé");
$$;