I modified the code, but still get the error. I am doing something wrong with XML::LibXML.

XML::LibXML->load: specify location, string, or IO at ./ServiceNowCMDB_ALL_CI_DumpXML.pl line 99.

#!/usr/bin/perl
use strict;
use warnings;
use 5.014;
use MIME::Base64;
use Data::Dumper;
use utf8;
use Text::Unidecode;
use XML::Twig;
use XML::LibXML;
use REST::Client;
use Data::Validate::Domain qw(is_hostname);
use Socket;
use Net::DNS;
use Net::MAC;
# Just in case we want to request JSON instead of XML from CMDB
#use JSON; Just in case we want to request JSON instead of XML from CM
+DB


my $scriptversion = "1.01 (Vance Turner, Staples Inc., 1/20/2016)";
 
our $header_count = 0;
our $fqdn_pos = 0;
our $ip_address_pos = 0;
our $dns_domain_pos = 0;
our $name_pos = 0;
our $mac_address_pos = 0;

# declare our command line options variables.
our $host = 'https://staplessb.service-now.com';
our $wanthelp = 0;
our $record_limit = 1;
our $class = '';
our $user = '';
our $pwd = '';
our $offset = 0;
our $outputfile = 'XMLDUMP.xml';

sub usage
{
    my $err = shift and select STDERR;
    print <<EOU;
usage: perl $0 [ -t URL ] [-r <max_records>] [-c <class>] [-u username
+ ] [-p password] [-o <outputfilename>]

    -t <host_url>         use <host_url> for the CMDB connection to op
+en. (https://staplessb.service-now.com)
    -r <record_limit>     use <record_limit> as the maximum nomber of 
+CMDB records to retrieve, default = '100'
    -c <class>            use <class> as the CMDB class records you wa
+nt to retrieve.
    -u <username>         CMDB username to use
    -p <password>         CMDB password to use
    -o <outputfilename>   write output to file named <outputfilename>,
+ defaults
                          to stdout if no outputfile is specified 

Examples:
   perl $0 -u user -p password -r max_records -o outputfile.xml   (cre
+ates outputfile.xml)

EOU
    exit $err;
} # usage

sub Get_CMDB_CI
{ 
my $client = REST::Client->new(host => $host);
my $encoded_auth = encode_base64("$user:$pwd", '');
$client->GET("/api/now/table/cmdb_ci?sysparm_limit=$record_limit&syspa
+rm_offset=$offset",
                {'Authorization' => "Basic $encoded_auth",
                 'Accept' => 'application/xml'});
my $CI_RECS = $client->responseContent();
return ($CI_RECS);
}


use Getopt::Long qw(:config bundling nopermute passthrough);

GetOptions (
    "help|h|?"    => \$wanthelp ,
    "t=s"    => \$host,
    "r=i"    => \$record_limit,
    "c=s"    => \$class,
    "u=s"    => \$user,
    "p=s"    => \$pwd,
    "o=s"    => \$outputfile,
    );

if ($wanthelp) {
  usage(1);
}

if (-s $outputfile) {
    print STDERR "File '$outputfile' already exists. Overwrite? [y/N] 
+> N\b";
    scalar <STDIN> =~ m/^[yj](es|a)?$/i or exit;
    }
 

my $input_xml;

open my $fh, '>>:encoding(utf8)', $outputfile or die "Cannot open: $ou
+tputfile: $!";

do {
    my $dom = XML::LibXML->load_xml(string=>$input_xml);
    my $isEmpty = !$dom->documentElement->hasChildNodes;
    last if $isEmpty;
    say $fh $input_xml;
    $offset = $offset + $record_limit +50000 ;
    print "Offset=$offset\n";
} while ($input_xml = Get_CMDB_CI());
close($fh) || warn "close failed: $!";
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Found the problem. The while loop may not have a value populated, and the do has to await new data. Testing that when the payload is empty it terminates with the exit, since last causes an error.

do {
    if ( $input_xml ) {
        $dom = XML::LibXML->load_xml( string => $input_xml );
        my $isEmpty = !$dom->documentElement->hasChildNodes();
        last if $isEmpty;
        say $fh $input_xml;
        $offset = $offset + $record_limit;
        say $offset;
    }
} while ($input_xml = Get_CMDB_CI());
close($fh) || warn "close failed: $!";
[download]