Thanks to all of you, your comments are really helpful to me. The background of my question is, that I am at a point where my scripts are getting more numerous and complex, while I am still just repeating what has been working in the past without necessarily understanding why. To be able to make a step ahead I dearly need to clarify some basic concepts. These seem obvious when I read about them, but get mangled when I try to apply them. I am just not comfortable with (de-) referencing complex data structures and testing for existence.

Part of my confusion seems to boil down to the following:
All of you write my hash structures like this 

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

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and as I understand the two following notations are equivalent 
if (not exists ${$coordinates}{$group}{$id}{$stage}{"coords"}) {
if (not exists ${${${${$coordinates}{$group}}{$id}}{$stage}}{"coords"}
+)

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All three work, they are dereferencing the hash reference $coordinates and provide the hash reference of the anonymous subhash that has at its top level the coordinate numbers as keys.

The following tests for the $coordinates-> notation also go through without error: 

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

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However, what about %{$$coordinates{$group}{$id}{$stage}{"coords"}}that is used in 
foreach my $coord_no (keys %{$$coordinates{$group}{$id}{$stage}{"coord
+s"}}) {…}

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I always thought this is equivalent and also would give the hash reference of the anonymous subhash? But 
if (not exists %{$$coordinates{$group}{$id}{$stage}{"coords"}}) {
exists argument is not a HASH or ARRAY element

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Thus, what does it return? And why is what it returns defined and true, but does not exist: 
if (not defined %{$$coordinates{$group}{$id}{$stage}{"coords"}}) {
if (!(%{$$coordinates{$group}{$id}{$stage}{"coords"}})) {

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both go through just fine.

To make my confusion complete, tests of a similar hash give different results: 

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

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Mmh.

In reply to Re: Test if a subhash in a referenced hash exists by Henri
in thread Test if a subhash in a referenced hash exists by Henri

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