my $response = $ua->post(
    'http://...',
    Content_Type => 'application/xml',
    Content      => $xml,
);

##</code><code>##

use HTTP::Common::Request qw( POST );

my $request = POST(
    "http://...",
    Content_Type => 'application/xml',
    Content      => $xml,
);

my $response = $ua->request($request);