#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML::Reader;

open my $TXT, '<', '1.txt' or die $!;
my %ids;
while (<$TXT>) {
    undef $ids{ (split /,/)[5] };
}
close $TXT;

my $xml = XML::LibXML::Reader->new( location => '1.xml' )
    or die "Cannot open xml\n";

while ($xml->nextElement('ALERT_ID')) {
    $xml->read;             # Go to the text value.
    my $id = $xml->value;

    if (exists $ids{$id}) {
        delete $ids{$id};

    } else {
        warn "Not found in csv: $id\n";
    }
}
print "$_\n" for keys %ids;