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YAML::XS - Perl YAML Serialization using XS and libyaml



    use YAML::XS;

    my $yaml = Dump [ 1..4 ];
    my $array = Load $yaml;


Kirill Simonov's libyaml is arguably the best YAML implementation. The C library is written precisely to the YAML 1.1 specification. It was originally bound to Python and was later bound to Ruby.

This module is a Perl XS binding to libyaml which offers Perl the best YAML support to date.

This module exports the functions Dump, Load, DumpFile and LoadFile. These functions are intended to work exactly like's corresponding functions. Only Load and Dump are exported by default.


  • $YAML::XS::LoadBlessed (since v0.69)

    Default: false.

    The default was changed in version 0.81.

    When set to false, it will not bless data into objects, which can be a security problem, when loading YAML from an untrusted source. It will silently ignore the tag and just load the data unblessed.

    In PyYAML, this is called SafeLoad.

    If set to true, it will load the following YAML as objects:

        local: !Foo::Bar [a]
        perl: !!perl/hash:Foo::Bar { a: 1 }
        regex: !!perl/regexp:Foo::Bar pattern

    You can create any kind of object with YAML. The creation itself is not the critical part. If the class has a DESTROY method, it will be called once the object is deleted. An example with File::Temp removing files can be found at

  • $YAML::XS::ForbidDuplicateKeys (since 0.84)

    Default: false

    When set to true, Load will die when encountering a duplicate key in a hash, e.g.

        key: value
        key: another value

    This can be useful for bigger YAML documents where it is not that obvious, and it is recommended to set it to true. That's also what a YAML loader should do by default according to the YAML specification.

  • $YAML::XS::UseCode

  • $YAML::XS::DumpCode

  • $YAML::XS::LoadCode

    If enabled supports deparsing and evaling of code blocks.

    Note that support for loading code was added in version 0.75, although $LoadCode was documented already in earlier versions.

  • $YAML::XS::QuoteNumericStrings

    When true (the default) strings that look like numbers but have not been numified will be quoted when dumping.

    This ensures leading that things like leading zeros and other formatting are preserved.

  • $YAML::XS::Boolean (since v0.67)

    Default: undef

    When used with perl 5.36 or later, builtin booleans will work out of the box. They will be created by Load and recognized by Dump automatically (since YAML::XS 0.89).

        say Dump({ truth => builtin::true });
        # truth: true

    For older perl versions you can use the following configuration to serialize data as YAML booleans:

    When set to "JSON::PP" or "boolean", the plain (unquoted) strings true and false will be loaded as JSON::PP::Boolean or objects. Those objects will be dumped again as plain "true" or "false".

    It will try to load [JSON::PP] or [boolean] and die if it can't be loaded.

    With that it's possible to add new "real" booleans to a data structure:

        local $YAML::XS::Boolean = "JSON::PP"; # or "boolean"
        my $data = Load("booltrue: true");
        $data->{boolfalse} = JSON::PP::false;
        my $yaml = Dump($data);
        # boolfalse: false
        # booltrue: true

    It also lets booleans survive when loading YAML via YAML::XS and encode it in JSON via one of the various JSON encoders, which mostly support JSON::PP booleans.

    Please note that JSON::PP::Boolean and behave a bit differently. Ideally you should only use them in boolean context.

    If not set, booleans are loaded as special perl variables PL_sv_yes and PL_sv_no, which have the disadvantage that they are readonly, and you can't add those to an existing data structure with pure perl.

    If you simply need to load "perl booleans" that are true or false in boolean context, you will be fine with the default setting.

  • $YAML::XS::Indent (since v0.76)

    Default is 2.

    Sets the number of spaces for indentation for Dump.


Handling unicode properly in Perl can be a pain. YAML::XS only deals with streams of utf8 octets. Just remember this:

    $perl = Load($utf8_octets);
    $utf8_octets = Dump($perl);

There are many, many places where things can go wrong with unicode. If you are having problems, use Devel::Peek on all the possible data points.


You can find out (since v.079) which libyaml version this module was built with:

    my $libyaml_version = YAML::XS::LibYAML::libyaml_version();



  • YAML::Syck

  • YAML::Tiny

  • YAML::PP



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