use strict;
use warnings;

my @ids = (1, 3, 4, 12, 41, 43, 44);
my $id = 4;
my $prev_id = defined($ids[$id-1]) ? $ids[$id-1] : 'undef';
my $next_id = defined($ids[$id+1]) ? $ids[$id+1] : 'undef';

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