# Each city may have different tables, so trawl through all the
# cities to find available table names
my %tables;
for my $city (keys %data) {
   $tables{$_}{$city}=$data{$k}{$_} for keys $data{$k};
}
print "Which table? Choices are:\n",
   join(", ", sort keys %tables), ">\n";
chomp($table=<>);

print "Which city? Choices are:\n",
   join(", ", sort keys %{$tables{$table}}), ">\n";
chomp($city=<>);
...