CVE-2026-13221

Perl versions through 5.43.9 produce silently incorrect regular expression matches when an alternation of more than 65535 fixed string branches is compiled into a trie in Perl_study_chunk.

When such branches are combined into a trie, the delta between the first branch and the shared tail is stored in a 16-bit field. A branch count above 65535 overflows the field, and the trie's match decision table is truncated with no warning or error.

A pattern of this shape produces false positive matches (matching strings it should not) and false negative matches (failing to match strings it should). When such a pattern gates an access or filtering decision, the result is wrong.