Re-reading JSON::XS there was one key word I missed the first time around. Here is the relevant POD with my emphasis added.
$perl_scalar = decode_json $json_text
The opposite of encode_json: expects an UTF-8 (binary) string and tries to parse that as an UTF-8 encoded JSON text, returning the resulting reference. Croaks on error.
So I need to encode before passing it to decode_json(). Here is the working program
#!/usr/bin/perl
use v5.14;
use warnings;
use utf8::all;
use Encode;
use JSON::XS qw( decode_json );
my $wl = '{"creche": "crèche",
"¥": "£",
"₡": "волн"
}';
my $pattern_list = decode_json( encode("utf8", $wl) );
