From CGI.pm (v2.80, line 455 or so)

    # Some people want to have their cake and eat it too!
    # Uncomment this line to have the contents of the query string
    # APPENDED to the POST data.
    # $query_string .= (length($query_string) ? '&' : '') . $ENV{'QUER
+Y_STRING'} if defined $ENV{'QUERY_STRING'};
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So if you're willing to change CGI.pm every upgrade, there you go. Why it's done I'm afraid I don't know.