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