Let's look at your code with some hand calculations thrown in for fun.

#!/usr/bin/perl

my %h = (
word1 =>[2,3],
word2 =>[1,3],
word3 =>[1,2]
);
$h{word1} = [ $h{word2},$h{word3}]; # [[1,3],[1,2]]
$h{word2} = [ $h{word1},$h{word3}]; # [[[1,3],[1,2]],[1,2]]
$h{word3} = [ $h{word1},$h{word2}]; # [[[1,3],[1,2]],[[[1,3],[1,2]],[1
+,2]]]

print @{ @{ $h{'word3'} }[0] }[0], "\n";
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If you look at the data above, you'll see that you're dereferencing the structure twice. Well, when looking at the zeroeth element, you'll notice that your print statement above gives a reference to an array, specifically [1,3].

To print it, you will need to deref it one more time.

print @{ @{ $h{'word3'} }[0] }[0]->[0], "\n"; # prints "1"
print @{ @{ $h{'word3'} }[0] }[0]->[1], "\n"; # prints "3"
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When in doubt, walk through your data by hand; use Data::Dumper; inspect things.

Hope this helps,