my $file = 'Events.txt';
open $info, "<", $file or die "Couldn't open events file: $!";
open $xml, "<", 'XML_FILE.xml' or die "Couldn't open xml file: $!";
open $output, ">", 'OUTPUT.txt' or die "Coudn't open output file: $!";
my $found = 0; #Flag for indication
while(my $line = <$info>)  {
    @get_data = split ',' ,$line;
    $alert_ID = $get_data[5];        

    seek $xml, 0, 0; # Seek to the beginning, to scan from beginning f
+or each ID. 
    while(my $xml_line = <$xml>)  {
        if ($xml_line =~ m|<ALERT_ID>$alert_ID</ALERT_ID>|) {
            $found = 1; 
            last;
        }
    }
    if ( $found == 1 )  {
        print "$alert_ID Found.\n";
        $found = 0;
    }
    else  {
        print $output "@get_data";
    }
}

close $info;
close $xml;
close $output;
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If you don't want anything else to be considered from the XML file. You can remove the other lines(either using sed or grep) before executing the script. It would reduce the processing time significantly.