Have a look at the very useful References quick reference.

Also, the keys doc. The argument for keys must be a hash and has a % sigil. 

for my $key (keys %hash){
#...
}

for my $key (keys %{$hashref}){ # dereferencing a hash
#...
}

[download]
The exists doc tells you its argument is a hash element which will have a $ sigil. You need to read your error message 
exists argument is not a HASH or ARRAY element

[download]
as 
exists argument is neither a HASH element nor an ARRAY element

[download]
if (exists $hash{some_key}){
#..
}

if (exists $hashref->{some_key}){ # dereferencing a hash element using
+ an arrow
#..
}

[download]
An observation.
...what about %{$$coordinates{$group}{$id}{$stage}{"coords"}}...
The %{...} does the dereferencing, there's no need for the extra $ in $$. I prefer the -> for dereferencing (many monks don't) but it is often unnecessary (as in this case).

Have a look at the tutorial linked to above. I look at it at least once a week. :-)

update: When I'm have a fight with a convoluted data structure I try out the syntax on a simplified version and get that working first. And while you're doing that don't forget the mighty Data::Dumper. Good luck!

In reply to Re^2: Test if a subhash in a referenced hash exists by wfsp
in thread Test if a subhash in a referenced hash exists by Henri

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