# Assume the following input delimited be a newline
Chicago, USA
Frankfurt, Germany
Berlin, Germany
Washington, USA
Helsinki, Finland
New York, USA

1   while (<>) {
2     chomp;
3     my ($city, $country) = split /, /;
4     push @{$table{$country}}, $city;
5   }
6
7   foreach $country (sort keys %table) {
8     print "$country: ";
9     my @cities = @{$table{$country}};
10     print join ’, ’, sort @cities;
11     print ".\n";
12   }

In the first part, line 4 is the important one.  We’re going to have a hash, %table, whose keys are country names, and whose values are (references to) arrays of city names.  After acquiring a city and country name, the program looks up $table{$country}, which holds (a reference to) the list of cities seen in that country so far.