Well you know the position of $e? If so then fill it in with 10 0's the first time through. When you get to adding $x calculate its starting position and replace the first 10 zeros. Alternately don't contstruct the string until you know $a - $x.

my $a = "this";
my $b = "is";
my $c = "a";
my $d = "test";
my $x = "here is the data";
my $e = sprintf("%010d", len($a . $b . $c . $d . $x) + 10);
$str = "$a$b$c$d$e$x";
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That code is obviously shortened, but you get the idea. Also you could add len's instead of len'ing the concated whole, doesn't realy matter. The 10 is the buffer for your position info.