use feature 'say';
binmode STDOUT, 'encoding(utf-8)';

my $str = "\xFC\x90\x80\x80\x80\x80";
my $ok = utf8::decode( $str );

say $ok ? 'ok' : 'not ok';
say $str;

##</code><code>##

ok
Code point 0x10000000 is not Unicode, may not be portable at demo.pl line 8.
"\x{10000000}" does not map to utf8 at demo.pl line 8.
\x{10000000}

##</code><code>##

my $ok = utf8::decode($json);
Dump( $json );