in reply to Automatic creation of hash elements

You have run into Autovivification.

To avoid that, you can do something like: 

 for( grep {exists $CATEGORY_DATA{$_}} 1..10){
   if( $FD_NAME eq $CATEGORY_DATA{$_}{FIELD_NAME}) { 
    ...
   }
  }

[download]

     "As you get older three things happen. The first is your memory goes, and I can't remember the other two... " - Sir Norman Wisdom

In Section Seekers of Perl Wisdom