Plack is, of course, an interface layer ... it provides a simple, consistent interface for you to write to, and then a set of personality-modules which you select according to the environment.   (One of those is, “ordinary CGI.”)   If your hosting service does not and cannot provide FastCGI, and the omission is business-important for you, just move to a different provider and change where your DNS entry goes. . .

But remember that it might not actually matter.   Most operating systems are “lazy” about getting rid of recently-used code segments, in anticipation that the same programs are often used repeatedly.   (The notion of “locality of reference” also has a corollary with regard to which programs are being requested.)