MIME::Parser versions before 5.521 for Perl do not limit the number of message parts by default, which can lead to memory exhaustion. The default max_parts option was set to -1, disabling limits and allowing a parser to build entities for every message part regardless of quantity. Version 5.521 introduces default limits of max_parts to 1000 and max_depth to 64, mitigating this issue. Users can still disable these limits by setting these options to -1.