8.1.  Character Encoding

   JSON text exchanged between systems that are not part of a closed
   ecosystem MUST be encoded using UTF-8 [RFC3629].

   Previous specifications of JSON have not required the use of UTF-8
   when transmitting JSON text.  However, the vast majority of JSON-
   based software implementations have chosen to use the UTF-8 encoding,
   to the extent that it is the only encoding that achieves
   interoperability.

   Implementations MUST NOT add a byte order mark (U+FEFF) to the
   beginning of a networked-transmitted JSON text.  In the interests of
   interoperability, implementations that parse JSON texts MAY ignore
   the presence of a byte order mark rather than treating it as an
   error.

##</code><code>##

2.  JSON Grammar

   ... 

##</code><code>##

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my @json_tests = (
    '',
    'crap',
    '[]',
    '{}',
    "  []",
    "\t[]",
    "\x{feff}[]",
    qq<\x{feff}\t{"k":"v"}>,
);

for my $test (@json_tests) {
    _json_chars($test);
    my $clean_json = clean_json($test);
    _json_chars($clean_json);
    say '-' x 40;
}

sub clean_json {
    my ($json) = @_;

    return '' unless length $json;

    state $re = qr{(?x:
        ^
        (
            (?: \x{feff}| )
        )
        (
            [\x{20}\x{09}\x{0a}\x{0d}]*
            (?: false|null|true|\[|\{|" )
            .*
        )
    )};

    if ($json =~ $re) {
        my ($bom, $text) = ($1, $2);

        if ($bom eq '') {
            say "JSON good as is.";
        }
        else {
            $json = $text;
            say "JSON cleaned -- BOM removed.";
        }
    }
    else {
        say 'Invalid JSON! Nothing cleaned.';
    }

    return $json;
}

sub _json_chars {
    my ($json) = @_;

    if (! length $json) {
        say 'Zero-length JSON';
    }
    else {
        say 'JSON chars: ',
            join '-', map sprintf('%x', ord), split //, $json;
    }

    return;
}

##</code><code>##

Zero-length JSON
Zero-length JSON
----------------------------------------
JSON chars: 63-72-61-70
Invalid JSON! Nothing cleaned.
JSON chars: 63-72-61-70
----------------------------------------
JSON chars: 5b-5d
JSON good as is.
JSON chars: 5b-5d
----------------------------------------
JSON chars: 7b-7d
JSON good as is.
JSON chars: 7b-7d
----------------------------------------
JSON chars: 20-20-5b-5d
JSON good as is.
JSON chars: 20-20-5b-5d
----------------------------------------
JSON chars: 9-5b-5d
JSON good as is.
JSON chars: 9-5b-5d
----------------------------------------
JSON chars: feff-5b-5d
JSON cleaned -- BOM removed.
JSON chars: 5b-5d
----------------------------------------
JSON chars: feff-9-7b-22-6b-22-3a-22-76-22-7d
JSON cleaned -- BOM removed.
JSON chars: 9-7b-22-6b-22-3a-22-76-22-7d
----------------------------------------