if (not exists $coordinates->{$group}{$id}{$stage}{"coords"}) {

##</code><code>##

if (not exists ${$coordinates}{$group}{$id}{$stage}{"coords"}) {
if (not exists ${${${${$coordinates}{$group}}{$id}}{$stage}}{"coords"}) 
##</code><code>##

if (not defined $coordinates->{$group}{$id}{$stage}{"coords"}) {
if (!($coordinates->{$group}{$id}{$stage}{"coords"})) {
##</code><code>##

foreach my $coord_no (keys %{$$coordinates{$group}{$id}{$stage}{"coords"}}) {…}

##</code><code>##

if (not exists %{$$coordinates{$group}{$id}{$stage}{"coords"}}) {
exists argument is not a HASH or ARRAY element 

##</code><code>##

if (not defined %{$$coordinates{$group}{$id}{$stage}{"coords"}}) {
if (!(%{$$coordinates{$group}{$id}{$stage}{"coords"}})) {

##</code><code>##

if (not exists %{$$original_data{$group}{$id}{$stage}}) {
# exists argument is not a HASH or ARRAY element
if (not defined %{$$original_data{$group}{$id}{$stage}}) {  
# works
if (!(%{$$original_data{$group}{$id}{$stage}})) {
# Can't use an undefined value as a HASH reference