Insert at Head - Perl DSA

Insert a new first node by pointing it at the old head and then moving the head pointer.

Algorithm

Basic Implementation

basic.pl

use strict;
use warnings;

sub node {
    my ($value, $next) = @_;
    return { value => $value, next => $next };
}

sub render {
    my ($head) = @_;
    my @parts;
    my $cursor = $head;
    while (defined $cursor) {
        push @parts, $cursor->{value};
        $cursor = $cursor->{next};
    }
    return join(" -> ", @parts) . " -> null";
}

my $head = node(20, node(30, undef));
my $new_head = node(10, undef);
$new_head->{next} = $head;
$head = $new_head;
print render($head), "\n";

Head insertion changes only two references: the new node points at the old head, then head moves to the new node.

Step 1 - Old first node

Before insertion, head points at node(20).

Step 2 - New node links to old head

Set new.next to the old first node before moving head.

Step 3 - Head moves to the new node

The final chain has 10 first: 10 -> 20 -> 30 -> null.

Complexity

Time: O(1)
Space: O(1)

Implementation notes

Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.

old head The previous first node becomes the second node.

constant-time insert Only the new node and head pointer change.