Web::Simple versions before 0.037 for Perl do not escape single quotes in HTML::Tags output.

HTML::Tags and XML::Tags escape a plain string value by replacing only &, ", < and > with entities. A value interpolated into a single-quoted attribute, as in <a href='$value'>, can therefore close the attribute and add new ones, such as an event handler. Double-quoted attributes are not affected.

An attacker who controls a value that an application interpolates into a single-quoted attribute can inject script into the generated page.