$a + ++$a

##</code><code>##

0 + $a++

##</code><code>##

$a + $a++

##</code><code>##

#!/usr/bin/perl -w
use strict;

for $b (
	'$a++       ',
	'0    + $a++',
	'0    + ++$a',
	'++$a + $a',
	'$a   + ++$a',
	'$a   + $a++',
	'++$a + $a++',
	'$a++ + ++$a',
	'$a+=$a++'
) {
  $a = 0;
  print "$b\t=", eval($b), "\t\$a= $a\n";
}
__END__
# results:
$a++       	=0	$a= 1
0    + $a++	=0	$a= 1
0    + ++$a	=1	$a= 1
++$a + $a	=2	$a= 1
$a   + ++$a	=2	$a= 1
$a   + $a++	=1	$a= 1
++$a + $a++	=3	$a= 2
$a++ + ++$a	=2	$a= 2
$a+=$a++	=1	$a= 1

##</code><code>##

$a + $a + $a++ # = 0 (correct)
$a + $a + ++$a # = 1 (correct)
$a + $a++ + $a # = 2 (the trailing $a has no effect maybe?)
$a + ++$a + $a # = 3 (the ++ effects previous $a as before?)

##</code><code>##

$a + $a + ++$a + $a++ # = 2 (correct!)
0 + ++$a + $a++ # = 3 (d'oh!)