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sub add_line {
   my ($tree,@line) = @_;
...
     $tree = [ $first_element, @line ];
   }
}

sub get_elements {
   my ($tree,@folder_names) = @_;
...
   my $subtree  = $tree->{shift(@folder_names)};
   return get_elements($subtree,@folder_names);
}

$directories{ join("/",@folder_names } = \@fields;

my @fields = @{ $directory{ join("/",@folder_names) } };