If I understand you correctly you expect

  my $x = 10;
  for my $i (1 .. $x) {
    $x = 1;
    print $i;
  }
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to execute only once. That would be the case if the above was equivalent to

  my $x = 10;
  for (my $i = 1; $i <= $x; $i++) {
    $x = 1;
    print $i;
  }
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, but it isn't. Recall that the non-C-style for (aka foreach) is a list iterator. 1 .. $x is a list. Compare with   my @foo = 1 .. $x; If you'd now do   foreach (@foo) { ... } you wouldn't expect foreach to act any differently upon modification of $x.

(Afaik for (EXPR .. EXPR) is nowadays optimized to not build a list, but that's irrelevant from a language point of view.)

Hope I've helped,
ihb