Just for completeness. I now have a working example of how to change data in Sharepoint via Perl.

#!/usr/bin/perl

use strict;

use LWP::UserAgent;
use LWP::Authen::Ntlm;
use HTTP::Request::Common;
use JSON;
use Data::Dumper;

# config
  my $host = "mysite.local";
  my $user = 'domain\spadmin';
  my $pass = "iLikeMyBike";
  my $resource = "business/mytestsite";
  my $listid = "1bd8d1bc-e797-41ae-97c0-e3876406d0fe";
  my $itemindex = 20;

# globals
  my $digest;

# create useragent
  my $ua = LWP::UserAgent->new( keep_alive => 1);
  $ua->credentials("$host:443", "", $user, $pass);
  $ua->default_header('Accept' => "application/json;odata=verbose");
  $ua->timeout( 10 );

# get digest
  my $response = $ua->post( "https://$host/$resource/_api/contextinfo"
+ );
  if ($response->is_success)
  {
    my $json = decode_json $response->decoded_content;
    $digest = $json->{d}->{GetContextWebInformation}->{FormDigestValue
+};
  }
  else
  {
    die $response->status_line;
  }

# post data
  my $msg = '{"__metadata":{"type":"SP.Data.DocLibItem"},"Title":"This
+ is my testtitle"}';

  my $req = POST "https://$host/$resource/_api/lists('$listid')/items(
+$itemindex)",
    "Content-type"     => "application/json;odata=verbose",
    "X-HTTP-Method"    => "MERGE",
    "X-RequestDigest"  => $digest,
    "IF-MATCH" => "*",
    "Content"  => $msg;

  $response = $ua->request( $req );

  my $json = decode_json $response->decoded_content;
  print Dumper( $json );

  if ($response->is_success)
  {
    print "place to be :)\n";
  }
  else
  {
    die $response->status_line;
  }
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