#!/usr/bin/perl

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

for my $n (0, 2, 20, 200, 2000, 20000, 200000)
  {
  my @bytes = reverse sprintf('%016X', $n) =~ s/^0{8}(0{4}(00)?)?//r =
+~ /../g;
  print "$n => @bytes\n";
  }
[download]

Outputs:

0 => 00
2 => 02
20 => 14
200 => C8
2000 => D0 07
20000 => 20 4E
200000 => 40 0D 03 00
[download]

Well.... I'm not sure how your regex works BUT it works great for unsigned numbers :-)

#!/usr/bin/perl

use strict;
use warnings;

sub number2hexString {
        return my $output .= join ' ', map { join ' ', sprintf('%016X'
+, $_) =~ s/^0{8}(0{4}(00)?)?//r =~ /../g } @_;
}

print number2hexString(2,20,200,2000,20000,200000)."\n";
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It doesn't completely solve my problem as it does not work with unsigned signed numbers

UPDATE: sorry, I was confused, obviously the last sentence should read ... does not work with signed numbers