No, bash/curl nor Perl fails. The problem was between the keyboard and the chair, as allways.

After an "Oh shit" experience when I found out I had forgotten to define the scope for the token request it still did not work. After some (actually a lot) googling I found that I allso had to ad a resouce in the request. In the process learned about "jq" and came up with the following bash script:

#! /usr/bin/bash

token=`curl \
    -d grant_type=client_credentials \
    -d client_id=[client_id] \
    -d client_secret=[client_secret] \
    -d scope=https://graph.microsoft.com/.default \
    -d resource=https://graph.microsoft.com \
    https://login.microsoftonline.com/[tenant_id]/oauth2/token \
    | jq -j .access_token`

curl -X GET \
    -H "Authorization: Bearer $token" \
    -H "Content-Type: application/json" \
    https://graph.microsoft.com/v1.0/groups \
    | jq .
[download]

This does the job. Gets a token an uses that token to request a listing of groups.
Getting that I could use the curl2lwp site and came up with the following Perl script

#! /usr/bin/perl -W

use strict;
use Data::Dumper;
use JSON;
use Config::Simple;
use FindBin;
#use lib "$FindBin::Bin/../lib";
use LWP::UserAgent;

my %config;
Config::Simple->import_from("$FindBin::Bin/groups.cfg",\%config) or di
+e("No config: $!");

my $ua = LWP::UserAgent->new(
    'send_te' => '0',
);

sub login_app { #    {{{1
    my $url = "$config{'LOGIN_ENDPOINT'}/$config{'TENANT_ID'}/oauth2/t
+oken";
    my $r = HTTP::Request->new(
        'POST' => $url,
            [
                'Accept'        =>    '*/*',
                'User-Agent'    =>    'curl/7.55.1',
                'Content-Type'    =>    'application/x-www-form-urlenc
+oded'
            ],
            "grant_type=client_credentials&client_id=$config{'APP_ID'}
+&client_secret=$config{'APP_PASS'}&scope=$config{'GRAPH_ENDPOINT'}/.d
+efault&resource=$config{'GRAPH_ENDPOINT'}"
    );

    my $result = $ua->request($r);

    if ($result->is_success){
        return decode_json($result->decoded_content)
    }else{
        print Dumper $result;
        die $result->status_line;
    }
    
}#    }}}

sub fetch { #    {{{1
    my $token = shift;
    my $url = shift;
    my $r  = HTTP::Request->new(
        'GET' => $url,
            [
                'Accept'        => '*/*',
                'Authorization' => "Bearer $token",
                'User-Agent'    => 'curl/7.55.1',
                'Content-Type'  => 'application/json'
            ],
    );    
    my $result = $ua->request($r);
    if ($result->is_success){
        return decode_json($result->decoded_content)
    }else{
        print Dumper $result;
        die $result->status_line;
    }
    
}#    }}}

my $token_request = login_app();
print Dumper $token_request;

if ($$token_request{'access_token'}){
    my $url = "$config{'GRAPH_ENDPOINT'}/v1.0/groups";
    my $groups = fetch($$token_request{'access_token'}, $url);
    print Dumper $groups;
}
[download]

I think the data line in the request is kinda ugly. There is problably a better/neater way of doing that. But I'm satisfied for now

Thanks all for the help, appreciate it a lot!