'Programming' => { q/Expert's Guide to Scala/ },
[download]

use strict;
use warnings;

my %tgs = (

    'articles' => {
                      'Java' => 'How to Program in Java',
                      'Perl' => 'Perl Programming for Biologists',
                      'Python' => {
                          'Programming' =>  q/Expert's Guide to Scala/
+,
                      },
                  },

    'eBooks'  =>  {
                      'Linux' => q/Linux: A Beginner's Guide/,
                  },
);

print $tgs{articles}{Python}{Programming};
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The odd number of hash elements warnings arises from the fact that there is only one key? Thanks

Yes, a key without a value, specifically  { q/Expert's Guide to Scala/ },

my $hashref = { 'hasherefs', 'are', 'made', 'with', 'curly', 'braces', 'they', 'like', 'to', 'come', 'in', 'pairs' };

Here is another way to initialize , the rehohy way

my %tgs;
$tgs{articles}{Python}{Programming}{"Expert's Guide to Scala"}
   = undef; #d3

$tgs{articles}{Perl}
 = "Perl Programming for Biologists"; #d1

$tgs{articles}{Java}
 = "How to Program in Java"; #d1

$tgs{eBooks}{Linux}
 = "Linux: A Beginner's Guide"; #d1
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And wouldn't you know it, its also a way to access certain elements :)

See also references quick reference

#!"C:\perl\bin\perl.exe" -w
# pragma
use strict;
use warnings;
use diagnostics;

# includes
use Data::Dumper;
use Data::Diver qw( Dive );

# data
my ( $tgs, %tgs );
%tgs = (
         'articles' => {
                   'Java'   => 'How to Program in Java',
                   'Perl'   => 'Perl Programming for Biologists',
                   'Python' => { 'Programming' => q/Expert's Guide to 
+Scala/, },
         },
         'eBooks' => { 'Linux' => q/Linux: A Beginner's Guide/, },
);
$tgs = \%tgs;

# dive hash
print Dumper $tgs->{articles}->{Python}->{Programming};

# returns:
#
# $VAR1 = 'Expert\'s Guide to Scala';
#
# or
#
print Dumper Dive( $tgs, qw ( articles Python Programming ) );

# returns:
#
# $VAR1 = 'Expert\'s Guide to Scala';
1;
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Re #1057536:

print keys $tgs{articles}{Python}{Programming};
# Type of argument to keys on reference must be unblessed hashref or a
+rrayref ....
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As choroba commented, i changed anonymous hash to anonymous array, so no key pair value. 'Programming' => q/Expert's Guide to Scala/ , and then, print @{$tgs{articles}{Python}{Programming}};