use strict;
use warnings;

my @list;

# Creating a doubly linked list

while (<DATA>){
  push @list, $_
}
chomp @list;

&print_list();

sub print_list {
  my $format = "%5s -> %5s -> %5s\n";
  printf $format,'prev','data','next';
  for (my $i=0; $i<$#list; $i++) {
    printf $format, $i>0  ? $list[$i-1] : 'undef',
      $list[$i], $i < $#list - 1 ? $list[$i+1] : 'undef'
  }

}

__DATA__
the
quick
brown
fox
jumps
over
the
lazy
dog

##</code><code>##

 prev ->  data ->  next
undef ->   the -> quick
  the -> quick -> brown
quick -> brown ->   fox
brown ->   fox -> jumps
  fox -> jumps ->  over
jumps ->  over ->   the
 over ->   the ->  lazy
  the ->  lazy ->   dog
 lazy ->   dog -> undef