You're using a hash reference called $where, but you've only defined a hash (i.e. not a reference) named %where.

Update: you probably want:

     print $where{$_};
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Update2: scratch that update.

foreach (sort keys %count) {
      if ($count{$_} == 1) {
                print "user $_ is ONLY logged in at one place: ";
                print keys %{$where{$_}},"\n";  # print the single key
      } else {
                print "user $_ is logged in at: ";
                @where = sort keys %{$where{$_}};
                print "@where\n";
      }
}
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Since $where{$user} always contains a hash ref.