Let's say $var1 contains both single digit and multiple digit numbers.

$var1 = "8 15 4 13 5 4 12";

The following will add a leading 0 to the single digit numbers. In this example, %2 means a maximum of 2 characters.

$var1 = sprintf("%2d", $var1);
$var1 =~ tr/ /0/;

$var1 should now contain the following.

08
15
04
13
05
04
12