Interesting.

In older versions of Perl, I encountered the issue where attempting to add a value to an undefined hash element would generate a warning.

I see under my modern version of Perl, this is not an issue:

#!/usr/bin/perl -w

use strict;

my $TEST_KEY = 'abc';
my $TEST_VAL = 5;

my %TestHash = ();

{
    print "-----[ Addition to undef ]---------------\n";
    %TestHash = ();
    $TestHash{$TEST_KEY} += $TEST_VAL;

    print "-----[ Addition testing for undef ]------\n";
    %TestHash = ();
    if (defined $TestHash{$TEST_KEY})
    {
        $TestHash{$TEST_KEY} += $TEST_VAL;
    }
    else
    {
        $TestHash{$TEST_KEY} = $TEST_VAL;
    }
}

exit;

__END__

C:\Steve\Dev\PerlMonks\P-2013-10-13@2339-HashUndef>hashtest.pl
-----[ Addition to undef ]---------------
-----[ Addition testing for undef ]------
[download]

I think -- not 100% sure, but I think -- I am happy about this.