HTTP::Date versions before 6.08 for Perl allow CPU exhaustion via polynomial regex backtracking in parse_date.
parse_date() matches the date string against a chain of alternative regexes, and str2time() delegates to it. Several of these patterns place unbounded quantifiers next to each other before a trailing \s*$ anchor. A valid date prefix followed by a long interior run of digits, letters, or whitespace and a single trailing byte that defeats the final match forces the engine to repartition the run, giving polynomial (about quadratic) backtracking. A header value of a few tens of kilobytes runs for tens of seconds of CPU.
HTTP::Date parses timestamps such as HTTP Date, Expires, and Last-Modified headers, which commonly originate from untrusted sources. Any caller that passes an untrusted date header to str2time() or parse_date() can be driven to consume unbounded CPU, a denial of service.