HTML::Gumbo versions before 0.19 for Perl disclose heap memory via type confusion.
Support for the <template> element was added to libgumbo 0.10.0 in 2015, but the walk_tree function in lib/HTML/Gumbo.xs was not updated to support it. The element was treated as a text-node, where strlen() over-reads the heap block that the pointer addresses.
Any caller that runs parse() with the default format => 'string', or with format => 'tree', on input containing a <template> element serializes the over-read bytes into the returned result, disclosing bounded heap contents. format => 'callback' reaches a croak on the unhandled node type and is unaffected.
Upgrade to HTML-Gumbo 0.19 or later, which adds GUMBO_NODE_TEMPLATE to the container node types handled by walk_tree.