It looks like $data is undefined.

#!/bin/env perl
use strict;
use warnings;
use XML::Simple;

# From https://metacpan.org/pod/XML::Simple#QUICK-START
my $data =   {
    'logdir'        => '/var/log/foo/',
    'debugfile'     => '/tmp/foo.debug',
    'server'        => {
        'sahara'        => {
            'osversion'     => '2.6',
            'osname'        => 'solaris',
            'address'       => [ '10.0.0.101', '10.0.1.101' ]
        },
        'gobi'          => {
            'osversion'     => '6.5',
            'osname'        => 'irix',
            'address'       => [ '10.0.0.102' ]
        },
        'kalahari'      => {
            'osversion'     => '2.0.34',
            'osname'        => 'linux',
            'address'       => [ '10.0.0.103', '10.0.1.103' ]
        }
    }
};

print "XMLout with data defined:\n\n", xml_out($data), "\n\n";
#print "XMLout with data undefined:\n\n", xml_out(undef), "\n\n";

sub xml_out {
  my $data = shift;
  my $xmlout = XMLout($data, NoAttr => 1, RootName => undef, SuppressE
+mpty => 0);
  return '<xml version="1.0" encoding="UTF-8">' . "\n$xmlout</xml>";
}
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Running this produces no warning but if you uncomment the second print it does.