if the xml changes it change

The three listed changes produce semantically equivalent XML documents. For example, if you make any or all of those changes, the document will still validate perfectly fine against the same schema. If the OP doesn't have full control over the source of the XML, those changes might happen, and the XPath using just name() would break.

Even though TIMTOWTDI, I prefer writing this

my $xpc = XML::LibXML::XPathContext->new($doc);
$xpc->registerNs('p', 'http://www.crtp.it');
$xpc->registerNs('p2', 'http://www.crpt.com');
my $result = $xpc->findnodes("/p2:login/p:utenti[p:email='$email' and 
+p:password='$digestpass']/p:admin");
[download]

over this

my $result = $doc->findnodes(qq{
/*[ local-name()="login" and namespace-uri()="http://www.crpt.com" ]
/*[ local-name()="utenti" and namespace-uri()="http://www.crtp.it" ]
  [
    *[ local-name()="email" and namespace-uri()="http://www.crtp.it" a
+nd string(node())="$email" ]
    and
    *[ local-name()="password" and namespace-uri()="http://www.crtp.it
+" and string(node())="$digestpass" ]
  ]
/*[ local-name()="admin" and namespace-uri()="http://www.crtp.it" ]
});
[download]

because it's shorter, has less repetition, and uses the recommended API.

The idea is to keep the xpath stuff where it belong

In the first piece of code above, the XPath stuff is already where it belongs: in an XPath expression context. From the XPath spec:

Expression evaluation occurs with respect to a context. ... The context consists of:

• ...
• the set of namespace declarations in scope for the expression

Also, it's what is recommended by findnodes:

There are several possible ways to deal with namespaces in XPath:

• The recommended way is to use the XML::LibXML::XPathContext module to define an explicit context for XPath evaluation, in which a document independent prefix-to-namespace mapping can be defined.

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