$ret - $d != $ret is interesting. One would think it's the same as $d (in boolean context), but it isn't. By testing the effect of $d ($ret - $d != $ret) instead of $d itself, the loop can be ended sooner. 
my $ret = 1;

for my $d (1e-15, 1e-20) {
   printf("%g %d %d\n",
      $d,
      ( $d              )?1:0,
      ( $ret-$d != $ret )?1:0,
   );
}

[download]
1e-015 1 1   # Often equivalent.
1e-020 1 0   # But not when there's an underflow.

[download]

In reply to Re^2: Exponential Function Programming by ikegami
in thread Exponential Function Programming by Abulil

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