As jmacloue points out, it looks like you are trying to send JSON data, and the $post_data is not valid JSON, and you are not setting the content-type. I find it is often easier to create the data as a normal Perl structure and then use the JSON module to encode it: 
use LWP::UserAgent;
use JSON;

my $ua = LWP::UserAgent->new;
my $server_endpoint = "http://api.someserver.com/v1/";
my $req = HTTP::Request->new(POST => $server_endpoint);

$req->authorization_basic('myusername', 'mypassword');

my $post_data = {
    parm1 => $valueasscalar,
    parm2 => \1, # this gives you a proper JSON 'true'
};
my $json = encode_json($post_data);

$req->content_type('application/json');
$req->content($json);

my $resp = $ua->request($req);
# etc

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In reply to Re: Bad Request 4000: HTTP::Request->new(POST => $server_endpoint) by tangent
in thread Bad Request 4000: HTTP::Request->new(POST => $server_endpoint) by Trace On

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