Generally XML strongly implies "use XML::Twig". However, unless the XML you want to process is very large, in this case I'd suggest XML::TreeBuilder is most appropriate. In particular take a look a the look_down method of HTML::Element. Consider:

use strict;
use warnings;
use XML::TreeBuilder;

my $xml = <<XML;
<root>
    <a attr='1'>Contents of a - 1</a>
    <b>Contents of b</b>
    <a attr='2'>Contents of a - 2</a>
    <c>Contents of c</c>
    <d>Contents of d</d>
</root>
XML

my $root = XML::TreeBuilder->new ();

$root->parse ($xml);

print "All the 'a' elements\n";
for my $elt ($root->look_down ('_tag' => 'a')) {
    print $elt->as_XML (), "\n";
}

print "All the 'b' elements\n";
for my $elt ($root->look_down ('_tag' => 'b')) {
    print $elt->as_XML (), "\n";
}

print "All the 'a' elements with attr='2'\n";
for my $elt ($root->look_down ('_tag' => 'a', 'attr' => '2')) {
    print $elt->as_XML (), "\n";
}
[download]

Prints:

All the 'a' elements
<a attr="1">Contents of a - 1</a>

<a attr="2">Contents of a - 2</a>

All the 'b' elements
<b>Contents of b</b>

All the 'a' elements with attr='2'
<a attr="2">Contents of a - 2</a>
[download]