$client has a script that needed minimal module support, but wants to parse JSON. Couldn't find anything like YAML::Tiny (which I borrowed to remove the YAML dependency), so I hacked up this regex to parse and extract JSON. Doesn't handle unicode yet, but that wasn't a client requirement. 
#!/usr/bin/env perl

use Data::Dumper qw(Dumper);

my $FROM_JSON = qr{

(?&VALUE) (?{ $_ = $^R->[1] })

(?(DEFINE)

(?<OBJECT>
  (?{ [$^R, {}] })
  \{
    (?: (?&KV) # [[$^R, {}], $k, $v]
      (?{ # warn Dumper { obj1 => $^R };
     [$^R->[0][0], {$^R->[1] => $^R->[2]}] })
      (?: , (?&KV) # [[$^R, {...}], $k, $v]
        (?{ # warn Dumper { obj2 => $^R };
       [$^R->[0][0], {%{$^R->[0][1]}, $^R->[1] => $^R->[2]}] })
      )*
    )?
  \}
)

(?<KV>
  (?&STRING) # [$^R, "string"]
  : (?&VALUE) # [[$^R, "string"], $value]
  (?{ # warn Dumper { kv => $^R };
     [$^R->[0][0], $^R->[0][1], $^R->[1]] })
)

(?<ARRAY>
  (?{ [$^R, []] })
  \[
    (?: (?&VALUE) (?{ [$^R->[0][0], [$^R->[1]]] })
      (?: , (?&VALUE) (?{ # warn Dumper { atwo => $^R };
             [$^R->[0][0], [@{$^R->[0][1]}, $^R->[1]]] })
      )*
    )?
  \]
)

(?<VALUE>
  \s*
  (
      (?&STRING)
    |
      (?&NUMBER)
    |
      (?&OBJECT)
    |
      (?&ARRAY)
    |
    true (?{ [$^R, 1] })
  |
    false (?{ [$^R, 0] })
  |
    null (?{ [$^R, undef] })
  )
  \s*
)

(?<STRING>
  (
    "
    (?:
      [^\\"]+
    |
      \\ ["\\/bfnrt]
#    |
#      \\ u [0-9a-fA-f]{4}
    )*
    "
  )

  (?{ [$^R, eval $^N] })
)

(?<NUMBER>
  (
    -?
    (?: 0 | [1-9]\d* )
    (?: \. \d+ )?
    (?: [eE] [-+]? \d+ )?
  )

  (?{ [$^R, eval $^N] })
)

) }xms;

sub from_json {
  local $_ = shift;
  local $^R;
  eval { m{\A$FROM_JSON\z}; } and return $_;
  die $@ if $@;
  return 'no match';
}

while (<>) {
  chomp;
  print Dumper from_json($_);
}

[download]

-- Randal L. Schwartz, Perl hacker

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

In reply to JSON parser as a single Perl Regex by merlyn

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