#!/usr/bin/perl

use strict;
use warnings;   
    
while (<DATA>) {
    chomp;
    my $o = $_;
    next if /\D/ || /^$/;
                   substr $_ => -0, 0, 0 x (4 - length);
    $_ ++ if 50 <= substr $_ => -2, 2, "";
                   substr $_ => -1, 0, ".";
    print "$o / 1000 = $_\n";
}

__DATA__
12000
12678
5 
49 
50 
499
500
501



12000 / 1000 = 12.0
12678 / 1000 = 12.7
5 / 1000 = 0.0
49 / 1000 = 0.0
50 / 1000 = 0.1
499 / 1000 = 0.5
500 / 1000 = 0.5
501 / 1000 = 0.5