In the latest version of libio-socket-ssl-perl shipped with Ubuntu 16.04,

the SSL_VERIFY_PEER requires to be a constant instead of string. The error
message printed out is

   SSL_verify_mode must be a number and not a string at /usr/share/perl5/IO/Socket/SSL.pm line 2166.

Need to use IO::Socket::SSL for the SSL_VERIFY_PEER to be set

perl-xCAT/xCAT/Client.pm

@@ -14,6 +14,7 @@ if ($^O =~ /^aix/i) {
 }
 use IO::Handle;
 use MIME::Base64 qw(decode_base64);
+use IO::Socket::SSL;
 
 my $inet6support;
 if ($^O =~ /^aix/i) {  # disable AIX IPV6  TODO fix

xCAT-server/lib/xcat/plugins/docker.pm

@@ -35,7 +35,6 @@ use xCAT::MsgUtils;
 use Cwd;
 use xCAT::Usage;
 use JSON;
-#use Data::Dumper;
 
 my $verbose;
 my $global_callback;
@@ -1003,7 +1002,7 @@ sub init_async {
     $async = HTTP::Async->new(
         slots => $args{slots},
         ssl_options => {
-            SSL_verify_mode => "SSL_VERIFY_PEER",
+            SSL_verify_mode => SSL_VERIFY_PEER,
             SSL_ca_file => $ssl_ca_file,
             SSL_cert_file => $ssl_cert_file,
             SSL_key_file => $key_file,