If you have various Perl XML modules installed from CPAN, then you are bound to have the "expat" package installed as well, because a lot of the CPAN XML modules depend on this package.

The "expat" distribution includes a utility called "xmlwf", which, according to its man page, "determines if an XML document is well-formed". (The description also says "It is non-validating." But there's a chance that your input data is actually not well-formed, and if that's true, then xmlwf might give you a better idea where the problems are in the data.)

There are lots of command-line options for controlling what xmlwf does with your input data -- read the man page and give it a try.

Once you figure out what's wrong with the data, you can either complain to the data provider(s) with specific issues, or else cook up a perl script (not using an XML module) that will do surgical edits of the data to make it XML-parsible.