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  keys is preserved (JSON does not preserver object key ordering itself).
  
  =item array
  
  A JSON array becomes a reference to an array in Perl.
  
  =item string
  
  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.
  
  =item number
  
  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, this module 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 to a 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, this module only guarantees precision up to but not including
  the least significant bit.
  
  =item true, false
  
  These JSON atoms become C<JSON::true> and C<JSON::false>,
  respectively. They are overloaded to act almost exactly like the numbers
  C<1> and C<0>. You can check whether a scalar is a JSON boolean by using
  the C<JSON::is_bool> function.
  
  =item null
  
  A JSON null atom becomes C<undef> in Perl.
  
  =item shell-style comments (C<< # I<text> >>)
  
  As a nonstandard extension to the JSON syntax that is enabled by the
  C<relaxed> setting, shell-style comments are allowed. They can start
  anywhere outside strings and go till the end of the line.
  
  =back
  
  
  =head2 PERL -> JSON
  
  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.
  
  =over 4
  
  =item 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. This module can optionally sort the hash keys
  (determined by the I<canonical> flag), so the same data structure will
  serialise to the same JSON text (given same settings and version of
  the same backend), 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.
  
  =item array references
  
  Perl array references become JSON arrays.
  
  =item other references
  
  Other unblessed references are generally not allowed and will cause an
  exception to be thrown, except for references to the integers C<0> and
  C<1>, which get turned into C<false> and C<true> atoms in JSON. You can
  also use C<JSON::false> and C<JSON::true> to improve readability.
  
     encode_json [\0,JSON::true]      # yields [false,true]
  
  =item JSON::true, JSON::false, JSON::null
  
  These special values become JSON true and JSON false values,
  respectively. You can also use C<\1> and C<\0> directly if you want.
  
  =item blessed objects
  
  Blessed objects are not directly representable in JSON, but C<JSON::XS>
  allows various ways of handling objects. See L<OBJECT SERIALISATION>,
  below, for details.
  
  =item simple scalars
  
  Simple Perl scalars (any scalar that is not a reference) are the most
  difficult objects to encode: this module will encode undefined scalars as
  JSON C<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