It's got nothing to do with operand evaluation order. It has to do with pre-increment returning lvalues. 

f(++$i, ++$i)

[download]
is more or less equivalent to 
do {
   local @_;
   alias $_[0] = ++$i;
   alias $_[1] = ++$i;
   &f;
}

[download]

Since the pre-increment operator returns the variable itself and not a copy, the above is the equivalent to 

do {
   local @_;
   ++$i; alias $_[0] = $i;
   ++$i; alias $_[1] = $i;
   &f;
}

[download]

As you see, f() sees the same value for both arguments as both arguments are the same variable.

In reply to Re^2: Pre vs Post Incrementing variables by ikegami
in thread Pre vs Post Incrementing variables by SavannahLion

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