bytes              chars
36353935 ----> 36 35 39 35  ---->  6 5 9 5
                                   | | | |
       first 4 digits      reverse | | | |
595661 -------------> 5956 ------> 6 5 9 5


##</code><code>##

#! /usr/bin/perl
use warnings;
use strict;

use Test::More;

sub what {
    my ($input) = @_;
    my $unpacked = unpack "V", $input;
    my $result = join "", map chr hex, sprintf("%x\n", $unpacked) =~ /(..)/g;
    is $result, reverse substr $input, 0, 4;
}

what($_) for 1000 .. 2000,
             2e4, 3e5, 4e6, 5e7, 6e8, 7e9, 8e10, 9e11, 2 ** 64 - 3;

done_testing();


##</code><code>##

reverse($input) % 10_000


##</code><code>##

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,