Assuming you meant "signed" for the second case, here's revised code with extended test cases. 

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11136191
use warnings;

my @test = map 2 * 10 ** $_, 0 .. 16;
for my $n ( 0, @test, map -$_, @test )
  {
  my @bytes = reverse sprintf('%016X', $n) =~
    s/^0{8}(0{4}(00)?)?(?=[0-7])//r =~
    s/^F{8}(F{4}(FF)?)?(?=[89A-F])//r =~
    /../g;
  print "$n => @bytes\n";
  }

[download]

Outputs: 

0 => 00
2 => 02
20 => 14
200 => C8 00
2000 => D0 07
20000 => 20 4E
200000 => 40 0D 03 00
2000000 => 80 84 1E 00
20000000 => 00 2D 31 01
200000000 => 00 C2 EB 0B
2000000000 => 00 94 35 77
20000000000 => 00 C8 17 A8 04 00 00 00
200000000000 => 00 D0 ED 90 2E 00 00 00
2000000000000 => 00 20 4A A9 D1 01 00 00
20000000000000 => 00 40 E5 9C 30 12 00 00
200000000000000 => 00 80 F4 20 E6 B5 00 00
2000000000000000 => 00 00 8D 49 FD 1A 07 00
20000000000000000 => 00 00 82 DF E4 0D 47 00
-2 => FE
-20 => EC
-200 => 38 FF
-2000 => 30 F8
-20000 => E0 B1
-200000 => C0 F2 FC FF
-2000000 => 80 7B E1 FF
-20000000 => 00 D3 CE FE
-200000000 => 00 3E 14 F4
-2000000000 => 00 6C CA 88
-20000000000 => 00 38 E8 57 FB FF FF FF
-200000000000 => 00 30 12 6F D1 FF FF FF
-2000000000000 => 00 E0 B5 56 2E FE FF FF
-20000000000000 => 00 C0 1A 63 CF ED FF FF
-200000000000000 => 00 80 0B DF 19 4A FF FF
-2000000000000000 => 00 00 73 B6 02 E5 F8 FF
-20000000000000000 => 00 00 7E 20 1B F2 B8 FF

[download]

Note that for 200, C8 is a negative number, so the 00 is required to make it so that C8 00 is positive.

In reply to Re^7: Split any number into string of 8-bit hex values (=1 byte) by tybalt89
in thread Split any number into string of 8-bit hex values (=1 byte) by drsweety

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