Drush 8.0.3 problem starring 'simplexml' and 'Failed to get available update data'

I've recently installed the latest version of Drush as the port is to be out of date. It seems to work OK, but when I try to download from drupal.org I get this error:

 # drush dl
simplexml_load_file(): I/O warning : failed to load external entity  [warning]
"" Project.php:74
Failed to get available update data from                             [error]
Could not download requested project(s).                             [error]
I've drawn a blank with it, so thanks in advance for any assistance.
Resolved. This was a SSL Certificate Verification problem and due to the ca-root-nss certificate file not being visible. I rebuilt the port

setting the knob ETCSYMLINK Add symlink to /etc/ssl/cert.pem

Drush 8 works fine now and I can update my Drupal sites.

I think this issue might have occurred when I upgraded to a later version of Perl.