#!/usr/bin/perl

use strict;
use XML::LibXML 1.70;

# Parse the document. XML::LibXML 1.70 introduced the load_xml method,
# which is somewhat nicer than the pre-1.70 methods for parsing data.
my $doc = XML::LibXML->load_xml(IO => \*DATA) or die;

# In your original example, the building was "Building A", but there
# was no node called "Building A" in the sample XML. 
my $bldg   = 'Office';
my $floor  = '1st Floor';

# I've changed your query so that it selects a <node> element.
# Previously it drilled down further to select the text inside
# the node's <label> element. There didn't seem to be any reason
# to do that.
my $query  = "//node[label = '$bldg']/node[label = '$floor']";

# Here's one way to do it...
if( my ($node) = $doc->findnodes($query) )
{
    my $new_node  = $doc->createElement("node");
    my $new_label = $doc->createElement("label");
    $node->addChild($new_node);
    $new_node->addChild($new_label);
    $new_label->appendText('10.1.1.1');
}

# This way is a little more concise...
if( my ($node) = $doc->findnodes($query) )
{
    $node
        -> addNewChild(undef, 'node')
        -> addNewChild(undef, 'label')
        -> appendText('10.2.2.2');
}

# This outputs the XML nicely indented. If you don't care
# about indentation, just use print $doc->toString.
use XML::LibXML::PrettyPrint;
print XML::LibXML::PrettyPrint
    -> new ( element => { compact => [qw/label/] } )
    -> pretty_print($doc)
    -> toString;

__DATA__
<top>
    <nodes>
        <node>
            <label>Office</label>
            <node>
                <label>1st Floor</label>
            </node>
            <node>
                <label>2nd Floor</label>
            </node>
        </node>
    </nodes>
</top>