That's a feature of CGI - it's treating your post data as an ISINDEX script. Definitely not something you want to rely on:

#!/usr/local/bin/perl

use strict;
use warnings;

use CGI;
use Data::Dumper;

my $cgi = CGI->new(  '{
      "Image": {
          "Width":  800,
          "Height": 600,
          "Title":  "View from 15th Floor",
          "Thumbnail": {
              "Url":    "http://www.example.com/image/481989943",
              "Height": 125,
              "Width":  "100"
          },
          "IDs": [116, 943, 234, 38793]
        }
   }'
);

print Dumper( $cgi );


$cgi = CGI->new(  '{
      "Image": {
          "Width=":  800,
          "Height": 600,
          "Title":  "View from 15th Floor",
          "Thumbnail": {
              "Url":    "http://www.example.com/image/481989943",
              "Height": 125,
              "Width":  "100"
          },
          "IDs": [116, 943, 234, 38793]
        }
   }'
);

print Dumper( $cgi );

[download]

Your best bet is to provide a cgi_get_query method that deals with the data how you want to deal with it (as outlined in the docs for CGI::Application).