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While Mason can be used in any text generation context, it is most frequently used to create dynamic web sites. As you probably know, executing Perl (or anything else for that matter) as a CGI can be very slow. Mason, because it is not a small amount of code, can be sluggish when run as a CGI under heavy loads.
To that end, Mason has been designed to play nice when run under mod_perl. In fact, Mason has quite a number of features that make it nicely suited to running under mod_perl.