server.pl

#! /usr/bin/perl
use warnings;
use strict;

use Plack::Builder;
use Plack::Request;
use Data::Dumper;

my $APP = sub {
    my ($env) = @_;
    my $req = 'Plack::Request'->new($env);

    [200, [], [Dumper($req)]]
};

builder {
    mount '/' => $APP;
};
[download]

client.pl

#!/usr/bin/perl
use warnings;
use strict;

use Encode;
use Digest::SHA;
use MIME::Base64 qw{ encode_base64 };
use LWP::UserAgent;

my $ua = 'LWP::UserAgent'->new;
my $url = 'http://localhost:5000';

my $str = 'hello'; # no special chars, 1-127
my $sha256 = Digest::SHA::sha256('abc123'); # bytes 0-255
my $hmac = Digest::SHA::hmac_sha512('a message', 'a key'); # bytes 0-2
+55
my $unicode_string = "\N{GREEK SMALL LETTER ALPHA}\N{GREEK SMALL LETTE
+R BETA}ab123"; # unicode chars mixed with lower-ascii
my $buffer = ""; # buffer to concatenate above into and POST them
for($str, $sha256, $hmac, $unicode_string){
    $buffer .= Encode::encode('UTF-8', $_);
}
my $b64 = encode_base64($buffer);
my @HTTPHeader = (ABC => $b64);
my $response = $ua->post($url, @HTTPHeader, Content => $b64);  # <-- C
+hange to your liking.
use Data::Dumper; print Dumper $response->content;
...
[download]

Now, run the server in one terminal and the client in another.