use strict;
use warnings;
use JSON::XS qw( decode_json );
use Data::Dumper;

binmode(STDOUT,":encoding(UTF-8)");

sub _p{return pack('U',$_[0])};

my ($wl,$pattern_list);
#create utf8 decoded(perl internal utf8) JSON character.
$wl = '{"creche": "cr'._p(0xE8).'che",';
$wl.= '"'._p(0xA5).'" : "'._p(0xA3).'",';
$wl.= '"'._p(8353).'": "'._p(1074)._p(1086)._p(1083)._p(1085).'"';
$wl.= '}';

#example 1 of OP
sub ex1 {
    my $pattern_list;

    #$pattern_list = decode_json($wl); #Wide character in subroutine entry

    #$pattern_list = JSON::XS->new->utf8(1)->decode($wl);#Wide character in subroutine ent

    #no warning: it seems this module expects encoded utf8 but decoded utf8 by default
    $pattern_list = JSON::XS->new->utf8(0)->decode($wl);
}
#ex1();
#print Dumper $pattern_list;

#example 2
sub ex2 {
    use File::Slurp qw( read_file );
    use utf8::all;

    open my $fh, '>:encoding(UTF-8)', 'test_file2';
    print {$fh} $wl;
    close $fh;

    #here utf8::all failed to set Slurp's io layer
    my $buffer= read_file('test_file2');
    print utf8::is_utf8($buffer) ? "buffer:utf8 flagged\n" : "buffer:not utf8 flagged\n";

    #you get 'encoded utf8 bytes and that is default for JSON::XS
    $pattern_list = decode_json( $buffer);

    #pattern_list is encoded utf8 string, not decoded
    print utf8::is_utf8($pattern_list) ? "pattern:utf8 flagged\n" : "pattern:not utf8 flag

}
ex2();
print Dumper $pattern_list;