sub format_add{
  my ($x, $y) = @_;
  my $ret = $x + $y;
  my $digs = length $x;
  sprintf("%0${digs}d", $ret );
}

##</code><code>##

  $a=0.5;    print ++$a; #prints 1.5
  $a='a0.5'; print ++$a; #prints a6 
     #(so ++ ignores the dot and squashes the 0)
  $a='2a';   print ++$a; #prints 3 
     #(because 2a in numeric context returns 2)