I think this FAQ will help:

How can I expand variables in text strings?

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.

This is easy. Use Data::Postponed. I wrote this module to handle just this kind of task.

use Data::Postponed 'postpone_forever';

$a = '';
$str = postpone_forever( "blabla" . $a . "blabla" );
$a = "XXX";
print "$str\n"; # prints blablaXXXblabla
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[Edit. I originally goofed by using $x instead of $a. Sorry. That typo is gone now. I also made the string "$blabla${a}blala" nicer to read.]

Surely, that module isn't capable of reversing the concatenation of its arguments, and then performing it again, is it? Perhaps you meant something more like this:

$a = '';
$str = "blabla" . postpone_forever( $a ) . "blabla";
$a = "XXX";
print "$str\n";
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Oh, yeah, you're right. $a = postpone_forever( '' ) would have been ok as well as $str = postpone_forever( "blabla" ); $str .= $a; $str .= "blabla";. Anything like that would be ok.

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Another way is to use substr's ability to insert strings.

# $str is built past $e's position, with $e omitted.
# $x is now known
substr
    $str,
    length($a)+length($b)+length($c)+length($d),
    0,
    sprintf '%010d', length($x);
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Isn't that just a sum of lengths?

$w_pos = length($a) + length($b) + length($c)
    + length($d) + 10 # length of $e
    + ...
    + length($v); # everything up to $w
$e = sprintf("%010d", $w_pos);
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You're right, but in my situation the construction of that string is very complex!

I think eric256 in Re: Create string with a delay and Zaxo in Re: Create string with a delay are both on the right track, but eric256's "len($a . $b . $c . $d . $x)" represents a separate pass through the complexity. For Zaxo's solution, the second argument of the sprintf needs to be "length($str)+10", calculated just before $x is added.

Combining and extending the two approaches a little, I'd suggests

$str = $a . $b . $c .$d;
$xpos_spot = length( $str );
$e = '0000000000';
$xpos_len = length($e);
$str .= $e;
$str .= $f;
...
$str .= $w;
substr $str, $xpos_spot, $xpos_len, 
        sprintf('%${xpos_len}d',length($str));
$str .= $x;
...
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There's a way to set up a kind of pointer into the part of the string you want to modify later, one that you can assign to. It uses a reference to "substr" with pre-defined parameters (known as currying). See BrowserUK's excellent Re: lhs substr(): refs vs. scalars writeup.