you cannot place the username AND an hash reference in a array. what you need is a hash of hash so you can do

$usernames{username} = { key1 => value1, ...};
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lookup by username is trivial (my %otherkeys = %{$usernames{that_username}})
to lookup by ip you need a foreach loop:

foreach my $key (keys %usernames) {
  if($usernames{$key}->{Framed-Address} eq $ip_i_look_for) {
    print "ip $ip_i_look_for owned by user $key \n";
    last;
  }
}
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