use strict;
use warnings;

my @ids = (1, 3, 4, 12, 41, 43, 44);
my $id = 4;
my $prev_id = 'undef';
my $next_id = 'undef';

for (my $i = 0; $i < scalar(@ids); ++$i)
{
   if ($ids[$i] == $id)
   {
      $prev_id = $ids[$i-1] if ($i > 0);
      $next_id = $ids[$i+1] if ($i+1 < scalar(@ids));
      last;
   }
}

print qq(ID: $id => NEXT: $next_id => PREV: $prev_id\n);