Here is a quick example using XML::Twig to get you going. Of course, there are many ways to do this, and this may not be optimal for your purposes, but it's a start:

use strict;
use warnings;
use XML::Twig;

my $xfile = <<EOF;
<people>
    <person><name>Jane Doe</name><age>42</age></person>
    <person><name>John Doe</name><age>43</age></person>
    <person><name>Foo Bar</name><age>43</age></person>
</people>
EOF

my $t= new XML::Twig();
$t->parse($xfile);
my $people  = $t->root();
my @persons = $people->children('person');
for (@persons) {
    my $name = $_->first_child('name')->text();
    my $age  = $_->first_child('age' )->text();
    if ($age == 43) { print "$name is $age\n" }
}

__END__
John Doe is 43
Foo Bar is 43
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