use strict;
use warnings;

my $data = {};
$data->{foo} = { a => 1};
$data->{bar} = "";

if ( %{ $data->{foo} } ) { ... }   # ok, because $data->{foo} is a hashref
if ( %{ $data->{bar} } ) { ... }   # not ok, because $data->{bar} is empty

##</code><code>##

if ( ref($data->{bar}) eq "HASH" ) { ... }   # ok, even if $data->{bar} is empty/undef

##</code><code>##

    ...
    next unless ref($coordinates->{$group}{$id}{$stage}{"coords"}) eq "HASH";

    # print individual and coordinate information