Reading the docs for XML::Twig, I see that "get_xpath" actually:

           Return a list of elements satisfying the $xpath. $xpath is 
+an
           XPATH-like expression.
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Note where it says "XPATH-like". It goes on to say that only a subset of the XPATH abbreviated syntax is covered. It appears that checking for sub-elements is not part of the [] syntax that XML::Twig supports.

All is not (yet) lost, as it does say at the bottom of the entry for get_xpath that "you can use XML::Twig::XPath to get "findnodes" to use XML::XPath as the XPath engine, with full coverage of the spec." Thus, you may need to install a few more modules to get this to work (e.g., ensure XML::XPath is installed - XML::Twig::XPath is part of XML::Twig).

In my answer to your previous question, I worked around this by looking for both elements, and skipping the current fig element if it didn't have both, and then using the label element for the work. I'm not sure how that is not acceptable to you.