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JSON::PP - JSON::XS compatible pure-Perl module.


use JSON::PP;

# exported functions, they croak on error
# and expect/generate UTF-8

$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
$perl_hash_or_arrayref  = decode_json $utf8_encoded_json_text;

# OO-interface

$coder = JSON::PP->new->ascii->pretty->allow_nonref;

$json_text   = $json->encode( $perl_scalar );
$perl_scalar = $json->decode( $json_text );

$pretty_printed = $json->pretty->encode( $perl_scalar ); # pretty-printing

# Note that JSON version 2.0 and above will automatically use
# JSON::XS or JSON::PP, so you should be able to just:

use JSON;



JSON::XS 2.27 (~2.30) compatible.


JSON::PP had been inculded in JSON distribution (CPAN module). It was a perl core module in Perl 5.14.


This module is JSON::XS compatible pure Perl module. (Perl 5.8 or later is recommended)

JSON::XS is the fastest and most proper JSON module on CPAN. It is written by Marc Lehmann in C, so must be compiled and installed in the used environment.

JSON::PP is a pure-Perl module and has compatibility to JSON::XS.



Some documents are copied and modified from "FUNCTIONAL INTERFACE" in JSON::XS.


$json_text = encode_json $perl_scalar

Converts the given Perl data structure to a UTF-8 encoded, binary string.

This function call is functionally identical to:

$json_text = JSON::PP->new->utf8->encode($perl_scalar)


$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.

This function call is functionally identical to:

$perl_scalar = JSON::PP->new->utf8->decode($json_text)


$is_boolean = JSON::PP::is_bool($scalar)

Returns true if the passed scalar represents either JSON::PP::true or JSON::PP::false, two constants that act like 1 and 0 respectively and are also used to represent JSON true and false in Perl strings.


Returns JSON true value which is blessed object. It isa JSON::PP::Boolean object.


Returns JSON false value which is blessed object. It isa JSON::PP::Boolean object.


Returns undef.

See MAPPING, below, for more information on how JSON values are mapped to Perl.


This section supposes that your perl vresion is 5.8 or later.

If you know a JSON text from an outer world - a network, a file content, and so on, is encoded in UTF-8, you should use decode_json or JSON module object with utf8 enable. And the decoded result will contain UNICODE characters.

# from network
my $json        = JSON::PP->new->utf8;
my $json_text   = CGI->new->param( 'json_data' );
my $perl_scalar = $json->decode( $json_text );

# from file content
local $/;
open( my $fh, '<', '' );
$json_text   = <$fh>;
$perl_scalar = decode_json( $json_text );

If an outer data is not encoded in UTF-8, firstly you should decode it.

use Encode;
local $/;
open( my $fh, '<', '' );
my $encoding = 'cp932';
my $unicode_json_text = decode( $encoding, <$fh> ); # UNICODE

# or you can write the below code.
# open( my $fh, "<:encoding($encoding)", '' );
# $unicode_json_text = <$fh>;

In this case, $unicode_json_text is of course UNICODE string. So you cannot use decode_json nor JSON module object with utf8 enable. Instead of them, you use JSON module object with utf8 disable.

$perl_scalar = $json->utf8(0)->decode( $unicode_json_text );

Or encode 'utf8' and decode_json:

$perl_scalar = decode_json( encode( 'utf8', $unicode_json_text ) );
# this way is not efficient.

And now, you want to convert your $perl_scalar into JSON data and send it to an outer world - a network or a file content, and so on.

Your data usually contains UNICODE strings and you want the converted data to be encoded in UTF-8, you should use encode_json or JSON module object with utf8 enable.

print encode_json( $perl_scalar ); # to a network? file? or display?
# or
print $json->utf8->encode( $perl_scalar );

If $perl_scalar does not contain UNICODE but $encoding-encoded strings for some reason, then its characters are regarded as latin1 for perl (because it does not concern with your $encoding). You cannot use encode_json nor JSON module object with utf8 enable. Instead of them, you use JSON module object with utf8 disable. Note that the resulted text is a UNICODE string but no problem to print it.

# $perl_scalar contains $encoding encoded string values
$unicode_json_text = $json->utf8(0)->encode( $perl_scalar );
# $unicode_json_text consists of characters less than 0x100
print $unicode_json_text;

Or decode $encoding all string values and encode_json:

$perl_scalar->{ foo } = decode( $encoding, $perl_scalar->{ foo } );
# ... do it to each string values, then encode_json
$json_text = encode_json( $perl_scalar );

This method is a proper way but probably not efficient.

See to Encode, perluniintro.


Basically, check to JSON or JSON::XS.


$json = JSON::PP->new

Rturns a new JSON::PP object that can be used to de/encode JSON strings.

All boolean flags described below are by default disabled.

The mutators for flags all return the JSON object again and thus calls can be chained:

my $json = JSON::PP->new->utf8->space_after->encode({a => [1,2]})
=> {"a": [1, 2]}


$json = $json->ascii([$enable])

$enabled = $json->get_ascii

If $enable is true (or missing), then the encode method will not generate characters outside the code range 0..127. Any Unicode characters outside that range will be escaped using either a single \uXXXX or a double \uHHHH\uLLLLL escape sequence, as per RFC4627. (See to "OBJECT-ORIENTED INTERFACE" in JSON::XS).

In Perl 5.005, there is no character having high value (more than 255). See to "UNICODE HANDLING ON PERLS".

If $enable is false, then the encode method will not escape Unicode characters unless required by the JSON syntax or other flags. This results in a faster and more compact format.

JSON::PP->new->ascii(1)->encode([chr 0x10401])
=> ["\ud801\udc01"]


$json = $json->latin1([$enable])

$enabled = $json->get_latin1

If $enable is true (or missing), then the encode method will encode the resulting JSON text as latin1 (or iso-8859-1), escaping any characters outside the code range 0..255.

If $enable is false, then the encode method will not escape Unicode characters unless required by the JSON syntax or other flags.

JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
=> ["\x{89}\\u0abc"]    # (perl syntax, U+abc escaped, U+89 not)



$json = $json->utf8([$enable])

$enabled = $json->get_utf8

If $enable is true (or missing), then the encode method will encode the JSON result into UTF-8, as required by many protocols, while the decode method expects to be handled an UTF-8-encoded string. Please note that UTF-8-encoded strings do not contain any characters outside the range 0..255, they are thus useful for bytewise/binary I/O.

(In Perl 5.005, any character outside the range 0..255 does not exist. See to "UNICODE HANDLING ON PERLS".)

In future versions, enabling this option might enable autodetection of the UTF-16 and UTF-32 encoding families, as described in RFC4627.

If $enable is false, then the encode method will return the JSON string as a (non-encoded) Unicode string, while decode expects thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs to be done yourself, e.g. using the Encode module.

Example, output UTF-16BE-encoded JSON:

use Encode;
$jsontext = encode "UTF-16BE", JSON::PP->new->encode ($object);

Example, decode UTF-32LE-encoded JSON:

use Encode;
$object = JSON::PP->new->decode (decode "UTF-32LE", $jsontext);


$json = $json->pretty([$enable])

This enables (or disables) all of the indent, space_before and space_after flags in one call to generate the most readable (or most compact) form possible.

Equivalent to:



$json = $json->indent([$enable])

$enabled = $json->get_indent

The default indent space length is three. You can use indent_length to change the length.


$json = $json->space_before([$enable])

$enabled = $json->get_space_before

If $enable is true (or missing), then the encode method will add an extra optional space before the : separating keys from values in JSON objects.

If $enable is false, then the encode method will not add any extra space at those places.

This setting has no effect when decoding JSON texts.

Example, space_before enabled, space_after and indent disabled:

{"key" :"value"}


$json = $json->space_after([$enable])

$enabled = $json->get_space_after

If $enable is true (or missing), then the encode method will add an extra optional space after the : separating keys from values in JSON objects and extra whitespace after the , separating key-value pairs and array members.

If $enable is false, then the encode method will not add any extra space at those places.

This setting has no effect when decoding JSON texts.

Example, space_before and indent disabled, space_after enabled:

{"key": "value"}


$json = $json->relaxed([$enable])

$enabled = $json->get_relaxed

If $enable is true (or missing), then decode will accept some extensions to normal JSON syntax (see below). encode will not be affected in anyway. Be aware that this option makes you accept invalid JSON texts as if they were valid!. I suggest only to use this option to parse application-specific files written by humans (configuration files, resource files etc.)

If $enable is false (the default), then decode will only accept valid JSON texts.

Currently accepted extensions are:


$json = $json->canonical([$enable])

$enabled = $json->get_canonical

If $enable is true (or missing), then the encode method will output JSON objects by sorting their keys. This is adding a comparatively high overhead.

If $enable is false, then the encode method will output key-value pairs in the order Perl stores them (which will likely change between runs of the same script).

This option is useful if you want the same data structure to be encoded as the same JSON text (given the same overall settings). If it is disabled, the same hash might be encoded differently even if contains the same data, as key-value pairs have no inherent ordering in Perl.

This setting has no effect when decoding JSON texts.

If you want your own sorting routine, you can give a code referece or a subroutine name to sort_by. See to JSON::PP OWN METHODS.


$json = $json->allow_nonref([$enable])

$enabled = $json->get_allow_nonref

If $enable is true (or missing), then the encode method can convert a non-reference into its corresponding string, number or null JSON value, which is an extension to RFC4627. Likewise, decode will accept those JSON values instead of croaking.

If $enable is false, then the encode method will croak if it isn't passed an arrayref or hashref, as JSON texts must either be an object or array. Likewise, decode will croak if given something that is not a JSON object or array.

JSON::PP->new->allow_nonref->encode ("Hello, World!")
=> "Hello, World!"


$json = $json->allow_unknown ([$enable])

$enabled = $json->get_allow_unknown

If $enable is true (or missing), then "encode" will *not* throw an exception when it encounters values it cannot represent in JSON (for example, filehandles) but instead will encode a JSON "null" value. Note that blessed objects are not included here and are handled separately by c<allow_nonref>.

If $enable is false (the default), then "encode" will throw an exception when it encounters anything it cannot encode as JSON.

This option does not affect "decode" in any way, and it is recommended to leave it off unless you know your communications partner.


$json = $json->allow_blessed([$enable])

$enabled = $json->get_allow_blessed

If $enable is true (or missing), then the encode method will not barf when it encounters a blessed reference. Instead, the value of the convert_blessed option will decide whether null (convert_blessed disabled or no TO_JSON method found) or a representation of the object (convert_blessed enabled and TO_JSON method found) is being encoded. Has no effect on decode.

If $enable is false (the default), then encode will throw an exception when it encounters a blessed object.


$json = $json->convert_blessed([$enable])

$enabled = $json->get_convert_blessed

If $enable is true (or missing), then encode, upon encountering a blessed object, will check for the availability of the TO_JSON method on the object's class. If found, it will be called in scalar context and the resulting scalar will be encoded instead of the object. If no TO_JSON method is found, the value of allow_blessed will decide what to do.

The TO_JSON method may safely call die if it wants. If TO_JSON returns other blessed objects, those will be handled in the same way. TO_JSON must take care of not causing an endless recursion cycle (== crash) in this case. The name of TO_JSON was chosen because other methods called by the Perl core (== not by the user of the object) are usually in upper case letters and to avoid collisions with the to_json function or method.

This setting does not yet influence decode in any way.

If $enable is false, then the allow_blessed setting will decide what to do when a blessed object is found.


$json = $json->filter_json_object([$coderef])

When $coderef is specified, it will be called from decode each time it decodes a JSON object. The only argument passed to the coderef is a reference to the newly-created hash. If the code references returns a single scalar (which need not be a reference), this value (i.e. a copy of that scalar to avoid aliasing) is inserted into the deserialised data structure. If it returns an empty list (NOTE: not undef, which is a valid scalar), the original deserialised hash will be inserted. This setting can slow down decoding considerably.

When $coderef is omitted or undefined, any existing callback will be removed and decode will not change the deserialised hash in any way.

Example, convert all JSON objects into the integer 5:

my $js = JSON::PP->new->filter_json_object (sub { 5 });
# returns [5]
$js->decode ('[{}]'); # the given subroutine takes a hash reference.
# throw an exception because allow_nonref is not enabled
# so a lone 5 is not allowed.
$js->decode ('{"a":1, "b":2}');


$json = $json->filter_json_single_key_object($key [=> $coderef])

Works remotely similar to filter_json_object, but is only called for JSON objects having a single key named $key.

This $coderef is called before the one specified via filter_json_object, if any. It gets passed the single value in the JSON object. If it returns a single value, it will be inserted into the data structure. If it returns nothing (not even undef but the empty list), the callback from filter_json_object will be called next, as if no single-key callback were specified.

If $coderef is omitted or undefined, the corresponding callback will be disabled. There can only ever be one callback for a given key.

As this callback gets called less often then the filter_json_object one, decoding speed will not usually suffer as much. Therefore, single-key objects make excellent targets to serialise Perl objects into, especially as single-key JSON objects are as close to the type-tagged value concept as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not support this in any way, so you need to make sure your data never looks like a serialised Perl hash.

Typical names for the single object key are __class_whatever__, or $__dollars_are_rarely_used__$ or }ugly_brace_placement, or even things like __class_md5sum(classname)__, to reduce the risk of clashing with real hashes.

Example, decode JSON objects of the form { "__widget__" => <id> } into the corresponding $WIDGET{<id>} object:

# return whatever is in $WIDGET{5}:
   ->filter_json_single_key_object (__widget__ => sub {
         $WIDGET{ $_[0] }
   ->decode ('{"__widget__": 5')

# this can be used with a TO_JSON method in some "widget" class
# for serialisation to json:
sub WidgetBase::TO_JSON {
   my ($self) = @_;

   unless ($self->{id}) {
      $self->{id} =;
      $WIDGET{$self->{id}} = $self;

   { __widget__ => $self->{id} }


$json = $json->shrink([$enable])

$enabled = $json->get_shrink

In JSON::XS, this flag resizes strings generated by either encode or decode to their minimum size possible. It will also try to downgrade any strings to octet-form if possible.

In JSON::PP, it is noop about resizing strings but tries utf8::downgrade to the returned string by encode. See to utf8.



$json = $json->max_depth([$maximum_nesting_depth])

$max_depth = $json->get_max_depth

Sets the maximum nesting level (default 512) accepted while encoding or decoding. If a higher nesting level is detected in JSON text or a Perl data structure, then the encoder and decoder will stop and croak at that point.

Nesting level is defined by number of hash- or arrayrefs that the encoder needs to traverse to reach a given point or the number of { or [ characters without their matching closing parenthesis crossed to reach a given character in a string.

If no argument is given, the highest possible setting will be used, which is rarely useful.

See "SSECURITY CONSIDERATIONS" in JSON::XS for more info on why this is useful.

When a large value (100 or more) was set and it de/encodes a deep nested object/text, it may raise a warning 'Deep recursion on subroutin' at the perl runtime phase.


$json = $json->max_size([$maximum_string_size])

$max_size = $json->get_max_size

Set the maximum length a JSON text may have (in bytes) where decoding is being attempted. The default is 0, meaning no limit. When decode is called on a string that is longer then this many bytes, it will not attempt to decode the string but throw an exception. This setting has no effect on encode (yet).

If no argument is given, the limit check will be deactivated (same as when 0 is specified).

See "SSECURITY CONSIDERATIONS" in JSON::XS for more info on why this is useful.


$json_text = $json->encode($perl_scalar)

Converts the given Perl data structure (a simple scalar or a reference to a hash or array) to its JSON representation. Simple scalars will be converted into JSON string or number sequences, while references to arrays become JSON arrays and references to hashes become JSON objects. Undefined Perl values (e.g. undef) become JSON null values. References to the integers 0 and 1 are converted into true and false.


$perl_scalar = $json->decode($json_text)

The opposite of encode: expects a JSON text and tries to parse it, returning the resulting simple scalar or reference. Croaks on error.

JSON numbers and strings become simple Perl scalars. JSON arrays become Perl arrayrefs and JSON objects become Perl hashrefs. true becomes 1 (JSON::true), false becomes 0 (JSON::false) and null becomes undef.


($perl_scalar, $characters) = $json->decode_prefix($json_text)

This works like the decode method, but instead of raising an exception when there is trailing garbage after the first JSON object, it will silently stop parsing there and return the number of characters consumed so far.

JSON->new->decode_prefix ("[1] the tail")
=> ([], 3)


Most of this section are copied and modified from "INCREMENTAL PARSING" in JSON::XS.

In some cases, there is the need for incremental parsing of JSON texts. This module does allow you to parse a JSON stream incrementally. It does so by accumulating text until it has a full JSON object, which it then can decode. This process is similar to using decode_prefix to see if a full JSON object is available, but is much more efficient (and can be implemented with a minimum of method calls).

This module will only attempt to parse the JSON text once it is sure it has enough text to get a decisive result, using a very simple but truly incremental parser. This means that it sometimes won't stop as early as the full parser, for example, it doesn't detect parenthese mismatches. The only thing it guarantees is that it starts decoding as soon as a syntactically valid JSON text has been seen. This means you need to set resource limits (e.g. max_size) to ensure the parser will stop parsing in the presence if syntax errors.

The following methods implement this incremental parser.


$json->incr_parse( [$string] ) # void context

$obj_or_undef = $json->incr_parse( [$string] ) # scalar context

@obj_or_empty = $json->incr_parse( [$string] ) # list context

This is the central parsing function. It can both append new text and extract objects from the stream accumulated so far (both of these functions are optional).

If $string is given, then this string is appended to the already existing JSON fragment stored in the $json object.

After that, if the function is called in void context, it will simply return without doing anything further. This can be used to add more text in as many chunks as you want.

If the method is called in scalar context, then it will try to extract exactly one JSON object. If that is successful, it will return this object, otherwise it will return undef. If there is a parse error, this method will croak just as decode would do (one can then use incr_skip to skip the errornous part). This is the most common way of using the method.

And finally, in list context, it will try to extract as many objects from the stream as it can find and return them, or the empty list otherwise. For this to work, there must be no separators between the JSON objects or arrays, instead they must be concatenated back-to-back. If an error occurs, an exception will be raised as in the scalar context case. Note that in this case, any previously-parsed JSON texts will be lost.

Example: Parse some JSON arrays/objects in a given string and return them.

my @objs = JSON->new->incr_parse ("[5][7][1,2]");


$lvalue_string = $json->incr_text

This method returns the currently stored JSON fragment as an lvalue, that is, you can manipulate it. This only works when a preceding call to incr_parse in scalar context successfully returned an object. Under all other circumstances you must not call this function (I mean it. although in simple tests it might actually work, it will fail under real world conditions). As a special exception, you can also call this method before having parsed anything.

This function is useful in two cases: a) finding the trailing text after a JSON object or b) parsing multiple JSON objects separated by non-JSON text (such as commas).

$json->incr_text =~ s/\s*,\s*//;

In Perl 5.005, lvalue attribute is not available. You must write codes like the below:

$string = $json->incr_text;
$string =~ s/\s*,\s*//;
$json->incr_text( $string );



This will reset the state of the incremental parser and will remove the parsed text from the input buffer. This is useful after incr_parse died, in which case the input buffer and incremental parser state is left unchanged, to skip the text parsed so far and to reset the parse state.



This completely resets the incremental parser, that is, after this call, it will be as if the parser had never parsed anything.

This is useful if you want ot repeatedly parse JSON objects and want to ignore any trailing data, which means you have to reset the parser after each successful decode.

See to "INCREMENTAL PARSING" in JSON::XS for examples.



$json = $json->allow_singlequote([$enable])

If $enable is true (or missing), then decode will accept JSON strings quoted by single quotations that are invalid JSON format.


As same as the relaxed option, this option may be used to parse application-specific files written by humans.


$json = $json->allow_barekey([$enable])

If $enable is true (or missing), then decode will accept bare keys of JSON object that are invalid JSON format.

As same as the relaxed option, this option may be used to parse application-specific files written by humans.



$json = $json->allow_bignum([$enable])

If $enable is true (or missing), then decode will convert the big integer Perl cannot handle as integer into a Math::BigInt object and convert a floating number (any) into a Math::BigFloat.

On the contary, encode converts Math::BigInt objects and Math::BigFloat objects into JSON numbers with allow_blessed enable.

$bigfloat = $json->decode('2.000000000000000000000000001');
print $json->encode($bigfloat);
# => 2.000000000000000000000000001

See to "MAPPING" in JSON::XS aboout the normal conversion of JSON number.


$json = $json->loose([$enable])

The unescaped [\x00-\x1f\x22\x2f\x5c] strings are invalid in JSON strings and the module doesn't allow to decode to these (except for \x2f). If $enable is true (or missing), then decode will accept these unescaped strings.




$json = $json->escape_slash([$enable])

According to JSON Grammar, slash (U+002F) is escaped. But default JSON::PP (as same as JSON::XS) encodes strings without escaping slash.

If $enable is true (or missing), then encode will escape slashes.


$json = $json->indent_length($length)

JSON::XS indent space length is 3 and cannot be changed. JSON::PP set the indent space length with the given $length. The default is 3. The acceptable range is 0 to 15.


$json = $json->sort_by($function_name)
$json = $json->sort_by($subroutine_ref)

If $function_name or $subroutine_ref are set, its sort routine are used in encoding JSON objects.

$js = $pc->sort_by(sub { $JSON::PP::a cmp $JSON::PP::b })->encode($obj);
# is($js, q|{"a":1,"b":2,"c":3,"d":4,"e":5,"f":6,"g":7,"h":8,"i":9}|);

$js = $pc->sort_by('own_sort')->encode($obj);
# is($js, q|{"a":1,"b":2,"c":3,"d":4,"e":5,"f":6,"g":7,"h":8,"i":9}|);

sub JSON::PP::own_sort { $JSON::PP::a cmp $JSON::PP::b }

As the sorting routine runs in the JSON::PP scope, the given subroutine name and the special variables $a, $b will begin 'JSON::PP::'.

If $integer is set, then the effect is same as canonical on.


For developers.



    depth        => $depth,
    indent_count => $indent_count,


    text    => $text,
    at      => $at,
    ch      => $ch,
    len     => $len,
    depth   => $depth,
    encoding      => $encoding,
    is_valid_utf8 => $is_valid_utf8,


This section is copied from JSON::XS and modified to JSON::PP. JSON::XS and JSON::PP mapping mechanisms are almost equivalent.

See to "MAPPING" in JSON::XS.



A JSON object becomes a reference to a hash in Perl. No ordering of object keys is preserved (JSON does not preserver object key ordering itself).


A JSON array becomes a reference to an array in Perl.


A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON are represented by the same codepoints in the Perl string, so no manual decoding is necessary.


A JSON number becomes either an integer, numeric (floating point) or string scalar in perl, depending on its range and any fractional parts. On the Perl level, there is no difference between those as Perl handles all the conversion details, but an integer may take slightly less memory and might represent more values exactly than floating point numbers.

If the number consists of digits only, JSON will try to represent it as an integer value. If that fails, it will try to represent it as a numeric (floating point) value if that is possible without loss of precision. Otherwise it will preserve the number as a string value (in which case you lose roundtripping ability, as the JSON number will be re-encoded toa JSON string).

Numbers containing a fractional or exponential part will always be represented as numeric (floating point) values, possibly at a loss of precision (in which case you might lose perfect roundtripping ability, but the JSON number will still be re-encoded as a JSON number).

Note that precision is not accuracy - binary floating point values cannot represent most decimal fractions exactly, and when converting from and to floating point, JSON only guarantees precision up to but not including the leats significant bit.

When allow_bignum is enable, the big integers and the numeric can be optionally converted into Math::BigInt and Math::BigFloat objects.

true, false

These JSON atoms become JSON::PP::true and JSON::PP::false, respectively. They are overloaded to act almost exactly like the numbers 1 and 0. You can check wether a scalar is a JSON boolean by using the JSON::is_bool function.

print JSON::PP::true . "\n";
 => true
print JSON::PP::true + 1;
 => 1

ok(JSON::true eq  '1');
ok(JSON::true == 1);

JSON will install these missing overloading features to the backend modules.


A JSON null atom becomes undef in Perl.

JSON::PP::null returns unddef.


The mapping from Perl to JSON is slightly more difficult, as Perl is a truly typeless language, so we can only guess which JSON type is meant by a Perl value.

hash references

Perl hash references become JSON objects. As there is no inherent ordering in hash keys (or JSON objects), they will usually be encoded in a pseudo-random order that can change between runs of the same program but stays generally the same within a single run of a program. JSON optionally sort the hash keys (determined by the canonical flag), so the same datastructure will serialise to the same JSON text (given same settings and version of JSON::XS), but this incurs a runtime overhead and is only rarely useful, e.g. when you want to compare some JSON text against another for equality.

array references

Perl array references become JSON arrays.

other references

Other unblessed references are generally not allowed and will cause an exception to be thrown, except for references to the integers 0 and 1, which get turned into false and true atoms in JSON. You can also use JSON::false and JSON::true to improve readability.

to_json [\0,JSON::PP::true]      # yields [false,true]
JSON::PP::true, JSON::PP::false, JSON::PP::null

These special values become JSON true and JSON false values, respectively. You can also use \1 and \0 directly if you want.

JSON::PP::null returns undef.

blessed objects

Blessed objects are not directly representable in JSON. See the allow_blessed and convert_blessed methods on various options on how to deal with this: basically, you can choose between throwing an exception, encoding the reference as if it weren't blessed, or provide your own serialiser method.

See to convert_blessed.

simple scalars

Simple Perl scalars (any scalar that is not a reference) are the most difficult objects to encode: JSON::XS and JSON::PP will encode undefined scalars as JSON null values, scalars that have last been used in a string context before encoding as JSON strings, and anything else as number value:

# dump as number
encode_json [2]                      # yields [2]
encode_json [-3.0e17]                # yields [-3e+17]
my $value = 5; encode_json [$value]  # yields [5]

# used as string, so dump as string
print $value;
encode_json [$value]                 # yields ["5"]

# undef becomes null
encode_json [undef]                  # yields [null]

You can force the type to be a string by stringifying it:

my $x = 3.1; # some variable containing a number
"$x";        # stringified
$x .= "";    # another, more awkward way to stringify
print $x;    # perl does it for you, too, quite often

You can force the type to be a number by numifying it:

my $x = "3"; # some variable containing a string
$x += 0;     # numify it, ensuring it will be dumped as a number
$x *= 1;     # same thing, the choise is yours.

You can not currently force the type in other, less obscure, ways.

Note that numerical precision has the same meaning as under Perl (so binary to decimal conversion follows the same rules as in Perl, which can differ to other languages). Also, your perl interpreter might expose extensions to the floating point numbers of your platform, such as infinities or NaN's - these cannot be represented in JSON, and it is an error to pass those in.

Big Number

When allow_bignum is enable, encode converts Math::BigInt objects and Math::BigFloat objects into JSON numbers.


If you do not know about Unicode on Perl well, please check "A FEW NOTES ON UNICODE AND PERL" in JSON::XS.

Perl 5.8 and later

Perl can handle Unicode and the JSON::PP de/encode methods also work properly.

$json->allow_nonref->encode(chr hex 3042);
$json->allow_nonref->encode(chr hex 12345);

Reuturns "\u3042" and "\ud808\udf45" respectively.


Returns UTF-8 encoded strings with UTF8 flag, regarded as U+3042 and U+12345.

Note that the versions from Perl 5.8.0 to 5.8.2, Perl built-in join was broken, so JSON::PP wraps the join with a subroutine. Thus JSON::PP works slow in the versions.

Perl 5.6

Perl can handle Unicode and the JSON::PP de/encode methods also work.

Perl 5.005

Perl 5.005 is a byte sementics world -- all strings are sequences of bytes. That means the unicode handling is not available.

In encoding,

$json->allow_nonref->encode(chr hex 3042);  # hex 3042 is 12354.
$json->allow_nonref->encode(chr hex 12345); # hex 12345 is 74565.

Returns B and E, as chr takes a value more than 255, it treats as $value % 256, so the above codes are equivalent to :

$json->allow_nonref->encode(chr 66);
$json->allow_nonref->encode(chr 69);

In decoding,


The returned is a byte sequence 0xE3 0x81 0x82 for UTF-8 encoded japanese character (HIRAGANA LETTER A). And if it is represented in Unicode code point, U+3042.



We ordinary expect the returned value is a Unicode character U+3042. But here is 5.005 world. This is 0xE3 0x81 0x82.


This is not a character U+12345 but bytes - 0xf0 0x92 0x8d 0x85.


memory saving


Most of the document are copied and modified from JSON::XS doc.


RFC4627 (


Makamaka Hannyaharamitu, <makamaka[at]>


Copyright 2007-2013 by Makamaka Hannyaharamitu

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.