foo = [2, 3]
bar = [4, 5]
sum = Hash.new { |hash, key| hash[key] = key.first + key.last }
p sum[foo]
p sum[bar]
p sum

#!/usr/bin/perl

use strict;
use warnings;
use Tie::Hash::DarmstadtPM;

tie my %hash,'Tie::Hash::DarmstadtPM',sub{$_[0] + $_[-1]};

my $foo = [2,3];
my $bar = [4,5];

print $hash{$foo},"\n";
print $hash{$bar},"\n";

~/entwicklung 327> perl tie_hash.pl 
5
9