You're interested in the contents, but you're looking at the carrier - what you need is HTTP::Request, probably:

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

use HTTP::Request;

my $host = 'what.ever.com';
my $uri = "http://$host/path/to/resource";
my $request = HTTP::Request->new(POST => $uri, [
   Host => $host,
   'User-Agent' => 'Mozilla whatever...',
   Accept => 'text/xml,application/xml,...',
   'Accept-Language' => 'en-us,en;q=0.5',
   # you get the idea...
]);

print {*STDOUT} $request->as_string();

__END__
poletti@PolettiX:~/sviluppo/perl$ perl zer.pl
POST http://what.ever.com/path/to/resource
Accept: text/xml,application/xml,...
Accept-Language: en-us,en;q=0.5
Host: what.ever.com
User-Agent: Mozilla whatever...
[download]

And yes, if you have a WWW::Mechanize (say $mech) object, you can then simply call:

my $response = $mech->request($request);
[download]

You have probably to work out if cookies are automatically inserted or not, and act consequently.