#!/usr/bin/perl

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

sub parse_post_data($) {
   my $data_str = shift;
   return "" if ($data_str eq "");

   my @matches = $data_str =~ /\"([^=]*)\"\s*?=\s*?>\"([^"}]*)\"/g;
   my %matches_hash = @matches;

   if (%matches_hash) {
      #print Dumper(\%matches_hash) . "\n";
   }
   return %matches_hash;
}

my $first_submission='{"param1"=>"val1", "param2"=>"val2", "param3"=>{"param4"=>"value4", "param5"=>"value5"}, "param6"=>"model1[field1],model1[field2]"}';
my $second_submission='{"param1"=>"new_value1", "param2"=>"val2", "param3"=>{"param4"=>"new_value4", "param5"=>"value5"}, "param6"=>"model1[diff_field1],model1[field2]"}';

my %first_submission_data = parse_post_data($first_submission);
my %second_submission_data = parse_post_data($second_submission);

foreach my $key (keys %first_submission_data) {
   if ($first_submission_data{$key} ne $second_submission_data{$key}) {
      print "Value of $key is different. " .
         "It was '$first_submission_data{$key}', now it is '$second_submission_data{$key}'\n";
   }
}

# will output:
#   Value of param4 is different. It was 'value4', now it is 'new_value4'
#   Value of param6 is different. It was 'model1[field1],model1[field2]', now it is 'model1[diff_field1],model1[field2]'
#   Value of param1 is different. It was 'val1', now it is 'new_value1'