here is a "recipe" for post that works. Notice there are a few differences from your code. I'm not sure which is the problem, but I suspect it is the qw(POST) needs to be imported.

use HTTP::Request::Common qw(POST);
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#!/usr/bin/perl 
use warnings;
use strict;
require LWP::UserAgent;

# There are the LWP::UserAgent 
# and HTTP::Request modules, 
# which together allow you to make POST requests . Example 

use HTTP::Request::Common qw(POST);

my $url = 'http://www.somewhere.com/cgi-bin/formular.cgi';

my $req = POST $url,
        [
        Host         => 'myHost',
        Server       => 'Any Value',
        OtherField   => 'Some Value',
        OneMoreField => 'whatever',
        ]
        ;
# Just a debug to STDOUT  
print "HTTP-FullRequest-Header: \n";
print $req->headers->as_string() , "\n";
print "HTTP-FullRequest-Header-Content: \n";
print $req->content() ,"\n";

# Now send it use LWP::UserAgent;  
my $ua = LWP::UserAgent->new();
my $response = $ua->request($req);

if ( $response->is_error() )
   { print "Error-Code : ", $response->code() , "\n";
     print "Error-Message : ", $response->message() , "\n";
   } else { print $response->content() , "\n"; }
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