What you are looking at is the difference between post and pre decrement/increment. Note predecrement and preincrement will first do the increment/decrement to the variable and then used. Whereas, postincrement and postdecrement will use the variable first and on the way out increment or decrement. That is:

print $a++;  #prints $a and then increments $a (hence prints 0)
print ++$a;  #increment $a and THEN print $a (hence prints 1)
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The same can be said of the -- unary operator.

update:hv++ for answering what was meant (and well, I answered the wrong question)

-enlil