open(DB_RESULT,">/path/to/result/db/file");
open(DB_SRC,"/path/to/source/db/file");
while (<DB_SRC>){
  if($_ ~= /$user/){
    my @user_fields = split /:/,$_;
    $user_fields[2] = $user_fields[2] + $newtraffic; 
    my $newline = $user_fields[0].':'.$user_fields[1].':'.$user_fields[2];
    print DB_RESULT, $newline;
  } else {
    print DB_RESULT, $_;
  }
}
close(DB_SRC);
close(DB_RESULT);
my $result = `mv /path/to/result/db/file /path/to/source/db/file`;