How about something like:

c:\@Work\Perl\monks\henzen>perl -wMstrict -MData::Dump -le
"use JSON;
 ;;
 my $json = q{
  {
      \"method\": \"xxx\",
      \"backend\": {
          \"client\": \"mytest009\",
          \"pw\": \"sdkjfhsfjhKJH87\"
      },
      \"blah\": \"123\"
  }};
 my $data = decode_json($json);
 dd $data;
 ;;
 my %valid = map { $_ => 1 } qw(method backend);
 my @invalid = grep { ! exists $valid{$_} } keys %$data;
 die qq{invalid keys(s) present: @invalid} if @invalid;
"
{
  backend => { client => "mytest009", pw => "sdkjfhsfjhKJH87" },
  blah    => 123,
  method  => "xxx",
}

invalid keys(s) present: blah at -e line 1.
[download]

Of course, you have to know all the valid keys in advance, and it's only a top-level check, but maybe a starting point.