Sounds like a problem on their end. There is a function in php called htmlspecialchars. Perhaps it's time to submit a bug report?

If you're looking to fix it on your end you can either edit Frontier::Client's call method or create a proxy that filters the php error messages. A solution for the former looks something like this:

# in sub call replace
# my $content = $response->content;
# with this
(my $content = $response->content) =~ s/.*?(?=<\?xml)//s;
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