split /(\d+)/

##</code><code>##

sub natcomp {
    my @a = split /(\d+)/, $a;
    my @b = split /(\d+)/, $b;
    my $last = min(scalar @a, scalar @b)-1;
    my $cmp;
    for my $i (0 .. $last) {
        unless($i & 1) {  # even
            $cmp = lc $a[$i] cmp lc $b[$i] || $a[$i] cmp $b[$i] and return $cmp;
        }else {  # odd
            $cmp = $a[$i] <=> $b[$i] and return $cmp;
        }
    }
    return scalar @a <=> scalar @b;  # shortest array comes first
}

##</code><code>##

chomp(my @array = <DATA>);
$\ = "\n";
print for sort natcomp @array;
__DATA__

1
A
amstelveen
Amsterdam
Amsterdam40
Amsterdam40b
Amsterdam5
Amsterdamned
Chapter 1 Section 10
Chapter 1 Section 3
Chapter 10 Section 2
Chapter 2 Section 1