Remove the reference operator in front of the anonymous hash on line 14 and then change $$user_list to $user_list. If you *really* must dereference the anonymous hash reference then you'd do it like so

${ $user_list[$num_users - 1] }->{lastname} = $lastname;
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But you almost certainly don't want to do that. See perlreftut and perlref for more info.
HTH

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broquaint