It's not because of an optimization.

                              # $_     $_[0]  $_[1]  $_[0]  $_[1]
                              # -----  -----  -----  -----  -----
$_ = 1;                       # 1
do {                          #
   local @_;                  #
   alias $_[0] = $_+=2;       # 3      $_            3
   alias $_[1] = $_+=3;       # 6      $_     $_     6      6
   &f;
};
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Add $_++; to f and you'll see it print 7, 7 because $_[0] and $_[1] are aliased to $_.

It works that way because += returns its LHS as an lvalue.

>perl -le "$_=1; ($_+=2)+=3; print $_"
6
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