Officially, the behaviour is undefined. The following documents what actually happens. It boils down to the fact that pre-increments and pre-decrements leave the input variable on the stack while post-increments and post-decrements leave a copy of it. 

$a++ + ++$a
                                  $a  Stack
1. Push copy of $a on stack       10  10
2. Increment $a                   11  10
3. Increment $a                   12  10
4. Push $a on stack               12  10,$a
5. Add                            12  22

[download]
++$a + $a++
                                  $a  Stack
1. Increment $a                   11
2. Push $a on stack               11  $a
3. Push copy of $a on stack       11  $a,11
4. Increment $a                   12  $a,11
5. Add                            12  23

[download]

Note: These steps don't correspond one-for-one to the ops actually involved for the sake of simplicity.

In reply to Re: Prefix and postfix increment/decrement operator query by ikegami
in thread Prefix and postfix increment/decrement operator query by saurabh.hirani

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.