Create a two dimensional hash like this:

@cities = qw/ US:Miami AU:Melbourne US:Chicago /;

map {my($co,$ci)=split/:/,$_; $n{$co}{$ci}=1 } @cities;

foreach $x (sort keys %n) {
   foreach (sort keys %{$n{$x}}) {
        printf "$x - $_\n";
   }
}
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Which will print out:

AU - Melbourne
US - Chicago
US - Miami