$ perl -wle 'printf "%.2e\n", 0.000000124687'
1.25e-07

% perl -lwe 'print sprintf("%.2e", 0.000124687)+0'
0.000125

sub to_three_sig_digits {
    my $val = shift;
    if ($val =~ /^(0\.0*)(\d+)$/) {
        my $pre = $1;
        my $app = $2;
        return $val if length($app) <= 3;

        my $digits = substr($app, 0, 3);
        my $fourth = substr($app, 3, 1);
        $digits += 1 if $fourth >= 5;
        $val = $pre . $digits;
    } elsif ($val > 1) {
        $val = sprintf("%.2f", $val);
    } 
    return $val;
}

#!/usr/bin/perl

use strict;
use warnings;

my $signifikant=3;

my $z="10.000000128536";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="10.000001117885000001100000";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="0.000000195675";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="0.1000000195675";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="1000.1399999";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="1000.00099999";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="1000.00090000019999";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="1000.99999";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="1099.99999";
print "$z --> ".round_significant($z,$signifikant)."\n";

$z="9999.9999999";
print "$z --> ".round_significant($z,$signifikant)."\n";

##########################################################
##########################################################
sub round_significant
{
  my $zahl=shift;
  my $stelle=shift;

  if(my ($zv,$zn) = $zahl =~/^([\d\.]+0)([1-9]{$stelle})\d*?$/)
  {
   $zv.="0"x$stelle;
   $zn++;
   my $l=length($zn);
   substr($zv,-1*$l,$l,$zn);
   $stelle--;
   $zv=~s/(\.\d{$stelle,}[1-9]+)0+$/$1/;
   $zahl=$zv;
  }
  else
  { $zahl= sprintf("%.${stelle}f",$zahl); }

  return $zahl;
}