Thanks.

  (0.1 ** length($1)) could also be written:
 ("1E-" . length($1))

update: This is what I came up with:

$frac = shift;

$frac =~ s/(\d+).$/$1/;   # remove last digit & get fraction size
$frac +=  "1E-" . length($1);   # add 1E-(size) => 0.0001

print $frac;
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Works for "0.123999", "10.12399" and ".12399"

The result is weird when the number does not have a fractional part, but that is not my case.