I second theorbtwo's recommendation to use Vars(), but in quick testing it looks like you can declare a hash and pass a reference to it as an argument to ReadParse():

my %vars;
CGI::ReadParse(\%vars);
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I used %vars to illustrate it could have any name, but of course you could use %in if you needed for compatibility.

UPDATE: And clintp is right on both counts, by the way. So, if you are going to use ReadParse() you are probably best off declaring our %in; and forgetting about it. But "there's more than one way to do it," and all of that. :)

- Matt Riffle