yup thanks guys, i ended up using map :) works best, i think why it wasn't working is because perls sprintf calls the systems sprintf when dealing with floats
Try this code on for size

$a[0] = 5;
$a[1] = 3;
$a[3] = 15;
for($n = 0; $n < scalar (@a); $n++) {
  $b[n]=sprintf("%.2f", $a[n]);
}

print "$b[0]\n";
print "$b[1]\n";
print "$b[2]\n";
print "$b[3]\n";
[download]

the output of this is "5.00\n\n\n\n"...

Notice the $a2 is not defined, perhaps something with C's sprintf doesn't like how perl deals with undefined vars???
this is copied and pasted from a shell so there are no sytax probs here :P

cheers
Dave