Well, for one thing, note that you're including on the command-line a double-quoted string (the $xml), containing double-quotes. That doesn't tend to work well. Perhaps you should try using open to pass in the args as an array and grab the output?

@curl = ('/usr/local/bin/curl', '--silent', '-m', '120', '-d', $xml,
         'http://localhost/check.cgi');
open CURL, "-|", @curl;

@responsexml = <CURL>;
print @responsexml;
[download]

(untested)

As for the error you gave, there's nothing in the code you posted that would get near SAX (whether there's any surrounding code that may, you haven't said enough to guess). And I don't think curl uses it either. So presumably, that's coming from the server and has nothing (directly) to do with this code anyway.