That's not really a good way to do it. What happens when a new version of CGI::Application comes out that makes a change in query()? The code above will break. Instead, call the real query() to do the work using SUPER:

package CGI::Application::POST_MAX;
use base 'CGI::Application';

sub query {
  my $self = shift;
  if (not exists $self->{__POST_MAX_SET__}) {
    $CGI::POST_MAX=1024 * 100;  # max 100K posts
    $self->{__POST_MAX_SET__} = 1;
  }
  return $self->SUPER::query(@_);
}

1;
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Of course, this still isn't 100% future-proof since it will break if a future version of CGI::Application uses the hash key "__POST_MAX_SET__" but I'd consider that an acceptable risk.

But is this even necessary? Can't you just put this at the top of your CGI::Application class:

BEGIN { $CGI::POST_MAX = 1020 * 100; }

-sam