$sitehit in first line is not resolved. Use double quotes, instead of single ones (or in this case, simply don't use quotes).

Try to run the following and observe the _uri portion:

use Data::Dumper;
use HTTP::Request;
my $sitehit = "abcdefg";


my $req = new HTTP::Request POST => '$sitehit';
$req->content_type('application/x-www-form-urlencoded');
$req->content('siteurl=$incoming{\'site\'}');
print Dumper($req);
[download]

Compare with this:

use Data::Dumper;
use HTTP::Request;
my $sitehit = "abcdefg";


my $req = new HTTP::Request POST => "$sitehit";
$req->content_type('application/x-www-form-urlencoded');
$req->content('siteurl=$incoming{\'site\'}');
print Dumper($req);
[download]