#! /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();