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.
https://metacpan.org/release/LEEJO/CGI-5.00/changes
https://metacpan.org/release/LEEJO/CGI-4.72/source/lib/CGI.pm
https://github.com/leejo/CGI.pm/commit/824956fbb0c4eb77044ba2dc9180a30df12cb309.patch