There are so many ways to do this. Here's one:

use strict;
use warnings;

use Test::More tests => 1;

my $have = '6578616d706c652e6d65';
my $want = '6578616d706c652e6d650000';

my $mult    = 8;
my $padchar = '0';

my $padlen = length ($have) % $mult;
$have .= $padchar x ($mult - $padlen) if $padlen;

is $have, $want;
[download]

See also the FAQ.

without use any external module?

You mean like String::Pad for instance? Why not use the best and most extensive archive of modules on the net?