The problem boiled down to having a NaN (not a number) value in the JSON string:

use strict;
use warnings;
use Data::Dumper;
use JSON;

my $str = <<'END';
{
  "field" : NaN,
  "other" : "name"
}
END

print Dumper JSON::decode_json $str;

Running this code will generate the exception malformed JSON string, neither tag, array, object, number, string or atom, at character offset 14 (before "NaN,\n "other" : "n...")

After reading a bit about the subject it turns out that NaN is not part of the JSON specification and it was a mistake to include it in the JSON file. Similarly Infinite and -Infinite

NaN in JSON in JavaScript

The JSON parser of JavaScript cannot handle NaN either

This JavaScript code:

console.log( JSON.parse('{ "x" : NaN  }') );

will gives the follow error:

Uncaught SyntaxError: Unexpected token N 

In the other direction, if a JavaScript data structure has NaN in it as in the following example, it will be encoded (stringified) to be null

console.log(JSON.stringify({ "x" : NaN }, undefined, 2));

{
  "x": null
} 

null in JSON handled by perl

If the NaN value is correctly encoded in the JSON as null then the JSON parsers in perl can also handle it, and it is converted into undef

use strict;
use warnings;
use Data::Dumper;
use JSON;

my $str = <<'END';
{
  "field" : null,
  "other" : "name"
}
END

print Dumper JSON::decode_json $str;

Results in the following output:

$VAR1 = {
          'other' => 'name',
          'field' => undef
        };

Conclusion

The JSON parsers of Perl are correct in not parsing NaN. The file created by the Python script was invalid.

Comments

No - they are incorrect - I have a string this is JSON serialized, but it needs to be turned into actual JSON. NaN is a valid value. JavaScript is not a real programming language and I hope someone replaces it for the web soon.