You had another problem (besides the ones already mentioned) - unlike PASCAL, subroutines are not (naturally) lexically scoped. You can lexically scope them, but it looks (and feels) different. Try the following:

for my $source (sort traffic_combined keys %data)
        {    
            print "$source data transfer out: $data{$source}{$all}.\n"
+;
            my $traffic_machine = sub
            {
                $data{$source}{$b} <=> $data{$source}{$a}
            }
            
            for my $destination (sort $traffic_machine keys %{$data{$s
+ource}})
            {
                next if ($destination eq $all);
                print "$source -> $destination: $data{$source}{$destin
+ation}.\n"
+;
            }
        }
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That way, we're making the inner sort into a closure, which will keep track of the current $source. sort checks its first argument to see if it's a sub or a closure, making this useful.