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;
};
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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