If you really want the "add" sub to modify the contents of the original "%a" hash, you'd do it like this: 
sub add {

    my $a_ref2 = shift;
    $$a_ref2{modified}++;  # (update: I forgot to include this step)

    foreach (keys %$a_ref2) {
        print $_ . "  " . $$a_ref2{$_} . "\n";
    }
}

[download]
(note the double sigils on the hash ref: %$a_ref2 refers to the entire hash, and  $$a_ref2{$_} refers to a single hash element; another way to do the latter is $a_ref2->{$_})

In reply to Re: References on hashes! by graff
in thread References on hashes! by Ace128

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