Some remarks:

my $ok = utf8::decode($json); is not the best way to validate utf8. Because:

1. It only validates Perl's notion of utf-8. Which is not exactly what the Unicode Consortium says.

   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;
   [download]

   output:

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

2. It always decodes a string in place. So doing

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

   is not super useful

Second, according to JSON's docs, encode_json is equivalent to JSON->new->utf8->encode($perl_scalar), while to_json is JSON->new->encode($perl_scalar). I don't know how these modules work, but the presence/absence of ->utf8 probably makes some difference?