So, how does the JSON data look like, with and without the problematic array? Also show the relevant code.

From your description, I guess something like this for the JSON:

With the array:

{ 
  "foo" : 42, 
  "bar" : 4711, 
  "the_array" : [ 1,2,3 ] 
}
[download]

Without the array:

{ 
  "foo" : 42, 
  "bar" : 4711
}
[download]

I assume that you use JSON::XS or something with a similar API, so after a call to decode_json(), you have a hash reference like this:

$data={
  foo => 42,
  bar => 4711,
  the_array => [ 1,2,3 ]
}
[download]

Or like this:

$data={
  foo => 42,
  bar => 4711
}
[download]

If your code accesses the array as @{$data->{'the_array'}}, it will of course fail. Check with exists $data->{'the_array'} before accessing it.

Maybe the JSON generator writes "the_array" : null in the problematic case. In that case $data->{'the_array'} will exist in Perl, but it will be undef. Test with defined $data->{'the_array'} before accessing the array.

With recent perl versions, you can also "replace" a missing array with a default array using the defined-or operator: @{$data->{'the_array'}//['default','values','here']}.

Alexander