if there is a way to keep GET parameters from populating $cgi->param().

Why? Check the result of request_method() first, then decide on how you want to handle parameters.

Or, if you really insist on that strange behaviour, inherit from CGI and override param() and multi_param(). Something like this:

package StrangeCGI;

# completely untested!

use strict;
use warnings;
use parent 'CGI';

sub multi_param
{
    my ($sself,@args)=@_;
    if ($self->request_method() ne 'POST') {
        return;
    } else {
        return $self->SUPER::multi_param(@args);
    }
}

sub param
{
    my ($sself,@args)=@_;
    if ($self->request_method() ne 'POST') {
        return;
    } else {
        return $self->SUPER::param(@args);
    }
}

1;
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I thought I remembered there being a pragma or something that allowed that,

I don't think that was ever possible.

but I'm probably misremembering.

You probably are.

Alexander