Actually (and this was the point of confusion), I made $_ an alias to $i three times. I would have understood if it had acted like

for (scalar($i=3, $i=2, $i=1)) {
  print "$_ and $i\n";
}
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where each assignment happens in turn, because of the comma operator, and $_ is aliased to the final result ($i) once. But that didn't happen. I expected each assignment expression to yield the value of its right-hand side, but Perl evaluated it more like:

$i=3, $i=2, $i=1;
for ($i, $i, $i) {
  print "$_ and $i\n";
}
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The interesting part

I can get the effect I expected by doing this:

for (0+($i=3), 0+($i=2), $i=1) {
  print "$_ and $i\n";
}
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