Web::Simple versions before 0.036 for Perl do not escape values interpolated into double-quoted strings in HTML::Tags output. Under use HTML::Tags or use XML::Tags, a double-quoted string literal becomes a StringThing object that holds each interpolated value as a separate part. The escaping step passes such objects through, and stringify joins their interpolated parts as is. A value passed on its own, as in <p>, $value, </p>, is entity-escaped, but the same value in "Hello $value" is not. An attacker who controls a value that an application interpolates into a double-quoted string can inject script into the generated page.