Concatenate multiple PostreSQL JSON columns in a single JSON root object - json

Question

I have a table (preferences), with several JSON columns (design_prefs, country_prefs, view_prefs, payment_prefs, etc.)
design_prefs --> {"color":"red", "height":15}
country_prefs --> {"language":"en", "currency":"pound"}
view_prefs --> {"layout":"list", "sortby":"category1", "view_brand":1, "view_price":1}
etc.
I retrieve a single row, but only with the columns required by the web context. Maybe just design_prefs, or design_prefs AND country_prefs, or design_prefs AND payment_prefs or whatever combination.
In the application (using Perl, Mojolicious and Mojo::Pg) I convert each JSON column in a Hash (key/value dictionary object used in Perl). And then I concatenate the multiple resulting hashes in a big "preferences" hash object.
It is working, but I would like to do the concatenation in Postgres, so in Mojolicious I only need to do a "JSON to hash" process.
Looking for aggregation functions in Postgresql I have found ways to do something like this:
{"color":"red", "height":15 }, {"language":"en", "currency":"pound"}
But I need this:
{"color":"red", "height":15, "language":"en", "currency":"pound"}
JSON type is stored as text in Pg 9.3, including the initial and ending bracket characters, so direct text concatenation is not a way.
Any guidance is appreciated, even recommendation to forget the idea and keep using the application instead the database.