Here is a simple way to do it:

open(MYFILE, "$cityfile") || die "Could not open $cityfile: $!\n";
while(<MYFILE>) {
  chomp;
  ($city, $country) = split(/:/, $_);
  $place{$country} .= "$city#";
}
close(MYFILE); 
for $x (keys %place) {
  print "$x:\n";
  for (split(/#/, $place{$x})) {
    print "\t$_\n";
  }
}
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