Only note that if the file is big, and there is not a lot of different files to create, calling open everytime a file must be update can be a considerable overhead (I've been in the same situation just a few days before).

If the different files can be determinated before the loop, you can open all of them and the use the apropiate (i.e., keeping them in a hash). When they're not known until runtime (it was my case), I used a closure returning lexical filehandles from a hash. Something like that:

{
        my %handles; # Closure keeps the hash between function calls
        sub handle{
                my $city = shift;
                my $temp;
                unless ($handles{$city}){
                        open $temp, ">", "$city.txt"
                                or die "$!";
                        $handles{$city} = $temp;
                }
                return $handles{$city};
        }
}
[download]

It worked for me and reduced the execution time about a 50%. Hope it helps.