still can't seem to find the libxml header files.

That's because your include flag is wrong too. It should be -I/usr/include/libxml2 and hopefully you'll be sorted.

rats. That didn't work either. Thanks for the input. I have to run and get a firewall replaced. I'll check back in the morning.

Here's a very short sample showing correct compilation against libxml2 on CentOS 6:

$ wget http://www.xmlsoft.org/examples/io2.c
--2017-05-31 15:13:18--  http://www.xmlsoft.org/examples/io2.c
Resolving www.xmlsoft.org... 91.121.203.120
Connecting to www.xmlsoft.org|91.121.203.120|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1182 (1.2K) [text/plain]
Saving to: `io2.c'

100%[======================================>] 1,182       --.-K/s   in
+ 0s      

2017-05-31 15:13:18 (53.9 MB/s) - `io2.c' saved [1182/1182]

$ gcc -o xmltest -I/usr/include/libxml2 -lxml2 -lz -lm io2.c
$ ./xmltest 
<?xml version="1.0"?>
<root>content</root>
$
[download]

If you get the same then the problem is likely to be on the cpan side. But if you get something different then it isn't a perl problem at all and you'll need to fix your libxml2 installation.