As someone else already mentioned, you really should take a look at just exactly what the pre- and post-increment operators are doing and when.

Also, and even more importantly, you have made a major error in thinking that your code snippet from your comment above ($x = ++$x + $x++ + $x) is the same as in your OP, which was $x = $x + ++$x + $x++. They are entirely different.

Here is a simple program to demonstrate the point:

#!/usr/bin/perl

use strict;
use warnings;

my $x = 5;
$x = $x + ++$x + $x++;
print "x: $x\n";

my $y = 5; # avoid any contamination from first group.
$y = ++$y + $y++ + $y;
print "y: $y\n";

exit;

__END__

Output:

[~/perl/test]# ./prepost.pl 
x: 18
y: 20
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