For the example you posted and assuming you want $response->{Item}->{ID} to be an array ref, davido's solution is probably the easiest - just force XML::Simple to return an array regardless of the number of elements.

If you need a more generalized solution, take a look at the ref function. It will allow you to check references like this:

if( ref( $var ) eq 'ARRAY' )
{
    # $var is a ref to an array
}
[download]

HTH