It changes perl's undef, etc to null as necessary for JS -- we used to have to use the option but now it automatically happens (says in docs that the option has been disabled). 

On Feb 22, 2013, at 7:07, Mark Smith <notifications@github.com> wrote:

> In htdocs/tools/endpoints/contentfilters.bml:
> 
> > @@ -113,8 +113,7 @@
> >  
> >      # save incoming changes
> >      } elsif ( $mode eq 'save_filters' ) {
> > -        local $JSON::UnMapping = 1;
> > -        my $obj = JSON::jsonToObj( $POST{json} );
> This pattern is different from other jsonToObj calls. What does this UnMapping thing do and does it change how we should be caling from_json?
> 
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