%n   special: *stores* the number of characters output so far
        into the next variable in the parameter list

##</code><code>##

my $name   = ...; ## from user input
my $amount = ...;

printf "$name : \$%.02f\n", $amount;

# instead of
# printf "%s : \$%.02f\n", $name, $amount;

##</code><code>##

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