The XPATH expression (if your at the root) would be '/doc/a/b/c/@d'. Using XML::LibXML something like this should work:

#!/usr/bin/perl

use strict;
use warnings;

use XML::LibXML;

my $xml = <<_XML;
<doc>
  <a>
    <b>
      <c d="foo"/>
    </b>
  </a>
</doc>
_XML


my $parser = XML::LibXML->new();
my $doc    = $parser->parse_string( $xml );
my $val    = $doc->findvalue( '/doc/a/b/c/@d' );

print $val, "\n";
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But that's only going to work if there's one a, b, and c element. If you think you'll have multiple elements, you'll have to collect them and then iterate the nodes.