CGI versions before 5.00 for Perl allow denial of service via submitted values used as a regex for optgroup selection in scrolling_list.
When rendering option groups, scrolling_list marks the selected options by interpolating each selected value, by default taken from the request parameters, unescaped into a substitution pattern.
A client can make the request die with an invalid pattern, or tie up the process with one that backtracks heavily.
Upgrade to CGI 5.00 or later.