cannot be exploited
Depends on what you mean, I guess.. Check out the perldoc, and look at the %n format. You can set values.
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Suppose I have the following code:
my $name = ...; ## from user input
my $amount = ...;
printf "$name : \$%.02f\n", $amount;
# instead of
# printf "%s : \$%.02f\n", $name, $amount;
Now, if a clever hacker goes in and inputs
$name = (" " x 5000) . "%n";
then after the code runs, $amount will be set to 5000. This is a pretty rare set of circumstances, but still something to watch out for.
Update: see also Re: $#="%c"; possible bug