#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;

my $xml = '<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:element name="foo">
<xsl:value-of select="bar"/>
</xsl:element>
</xsl:stylesheet>';

my $dom = 'XML::LibXML'->load_xml(string => $xml) or die;
my $xpc = 'XML::LibXML::XPathContext'->new($dom);

for my $element ($xpc->findnodes('//xsl:element')) {
    my ($name_attr) = $xpc->findnodes('@name', $element);
    $element->removeAttribute('name');
    $element->setNodeName($name_attr->getValue);
}
print $dom;