use strict;
use XML::Rules;
my $filter = XML::Rules->new(
    style => 'filter',
    rules => {
        _default => 'raw',
        p => sub {
            my ($tag, $attr) = @_;
            if ($attr->{v} eq 'YES') {
                return $tag => $attr
            } else {
                return;
            }
        },
    },
);

$filter->filter(\*DATA);

__DATA__
<?xml version="1.0" encoding="UTF-8" ?>
<e>
    <p id="1" v="YES">
        <e1>The</e1>
        <e2>Bye</e2>
    </p>
    <p id="2" v="NO">
        <e1>Border</e1>
        <e2>Lamp</e2>
    </p>
</e>
[download]

Unlike the XML::LibXML and the presented XML::Twig solutions, this one doesn't keep the whole (original or resulting) document in memory, instead it only stores the contents of one <p> at any time. If the file is huge and contains a lot of small <p> tags, this may make a big difference.