One possible solution uses the XML::XPath-module and uses XPath expressions to traverse the xml-file.

• / selects the document root
• descendant-or-self:: selects all nodes in or below the root node (which was set using /)
• The node test comment() is true for any comment node.

use strict;
use warnings;

use XML::XPath;

my $xml = q|<?xml version="1.0" ?>
<xml><!-- A single line comment -->

<class_list>
    <student>
        <name>Robert</name>
        <!-- A comment deeply inside the file -->
        <grade>A+</grade>
    </student>
<!-- Here starts a multi line comment
    <student>
        <name>Lenard</name>
        <grade>A-</grade>
    </student>
-->
</class_list>
</xml>
|;

my $xp = XML::XPath->new(xml => $xml);

my $nodeset = $xp->find('/descendant-or-self::comment()');

foreach my $node ($nodeset->get_nodelist) {
    print "FOUND\n",
        $node->getValue,
        "\n";
}
[download]

FOUND
 A single line comment 
FOUND
 A comment deeply inside the file 
FOUND
 Here starts a multi line comment
    <student>
        <name>Lenard</name>
        <grade>A-</grade>
    </student>
[download]