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
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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;
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Now, if a clever hacker goes in and inputs

$name = (" " x 5000) . "%n";
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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