Consider the output of

  $m = 20;  print ++$m + $m++;
[download]

and

  $m = 20;  print noop(++$m) + $m++;

  sub noop{ return shift }
[download]

What's going on here?

If you'd like to see how arcane your knowledge of Perl really is, try guessing what this prints without reading ahead or evaluating the code.

This is trivially explained, and I'm amazed there are still people giving this any serious consideration. Let me spell it out once more, in easy to read letters:

Modifying the same variable using auto-increment/decrement more than once in the same statement gives undefined behaviour.

The first sentence in perlop.pod regarding auto-increment and auto-decrement reads:

"++" and "--" work as in C.

And there you have it. Works as in C. And in C, using it twice on the same variable in the same statement gives undefined behaviour.

Abigail

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