In case of GET, the parameters and their values are passed as environment variables to the CGI program in charge. In case of post, the parameters and values (i.e. the content body) are passed into the CGI program in charge via file handle 0 (STDIN). It can be retrieved with the standard mechanisms to read from STDIN:

my $content_body = do { local $/; <> };
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But you shouldn't do that yourself. The perl core module CGI handles that for you.