This is not working

Your code starts with Use REST::Client; when it should be use REST::Client; because Perl is case-sensitive and use needs to be all lowercase. It's no surprise it isn't working.

You may also need to set some headers depending on what the server expects. Here is a fully working SSCCE to get you started.

#!/usr/bin/env perl 
use strict;
use warnings;
use REST::Client;

my $rest = REST::Client->new ({
    host => 'http://httpbin.org'
});
my $data = '{"key":"value"}';
my $headers = {
    Accept => 'application/json',
    'Content-type' => 'application/json',
};
$rest->POST ('/anything', $data, $headers);
print $rest->responseContent;
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