Why not? Perl was the language of choice for the author. There are a number of reasons why this was a good choice. Perl is an interpreted language that has ports for almost every popular platform available today. This means that a user can run the same POPFile code without any changes or recompiles on any available Perl installation (unlike languages such as C). Perl also has very powerful text string functions built in, which makes it very well suited to email filtering.