Use of double negation (!!)

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It converts non-boolean types to boolean (dualvar(0,"") or 1).

It is a shortcut way of doing this, instead of trying to cast it explicitly (which may take more characters). The ! operator negates the truthness of its argument. Hence, two of them are used.

Many object types are "truthy", and others are "falsey".

  • The only false values are 0, undef, "", "0" and some overloaded objects.
  • Examples of true values are 1, "asdf", and all other values.

That is a lot of funk for a sort block!

It's essentially a two-level sort :

  1. ascii-betical
  2. deleted files first, then undeleted

So one could rewrite it as (untested):

@documents = sort {  exists $deleted_documents_names{$a} # same effect as '!!'
                       <=> 
                     exists $deleted_documents_names{$b}
                  ||
                     uc( $a ) cmp uc( $b )
                  }
             @documents;

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