%missingusers is the uninitialized value. If you look a little deeper you'll find out the real problem:

#!/usr/bin/perl -l

use strict;
use warnings;
use LWP::UserAgent;

my $ua = new LWP::UserAgent;
my $req = HTTP::Request->new(
    POST => 'https://api.twitter.com/1/users/lookup.json');
if ($ua->is_protocol_supported('https')) {
    print "HTTPS is allowed";
}
else {
    print "This is HTTP-only";
}

my $res = $ua->request($req);
if ($res->is_success) {
    print $res->as_string;
}
else {
    print "Failed: ", $res->status_line;
    print $res->decoded_content;
}
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