Please note how I kept the reverse and join outside for generic flexibility of use.

NB: you must still take care of negative $n!

use strict;
use warnings;
use feature "say";

sub hexbytes {
    my ($n)=@_;

    my $nibbles = $n
      ? int( log($n)/log 256 )+1
        : 1  ;                          # 00 has no log

    $nibbles *= 2;                      # 2 nibbles = 1 byte

    return sprintf( '%0*x', $nibbles, $n ) =~ /(..)/g;
}


say "$_ => ", join " ", reverse hexbytes($_) for 0,2,20,200,2000,20000
+,200000;
[download]

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/hex_reverse.pl 
0 => 00
2 => 02
20 => 14
200 => c8
2000 => d0 07
20000 => 20 4e
200000 => 40 0d 03

Compilation finished at Mon Aug 30 15:57:08
[download]