01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16
01 02 03 04 12 13 15 05 11 16 15 06 10 09 08 07

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
01 02 03 04 05 16 17 18 19 06 15 24 25 20 07 14 23 22 21 08 13 12 11 10 09

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;

my $n = $ARGV[0] || 5;
my @spiral = gen_spiral($n);

output(@spiral);

sub output {
    my @spiral = @_;
    my $n = sqrt @spiral;
    my $cnt;
    for ( @spiral ) {
        print "\n" if $cnt++ % $n == 0;
        printf("%-3d ", $_);
    }
}

sub gen_spiral {
    my $n = shift;
    my ($tot, @spiral) = ($n * $n, ());
    my ($index, $cnt, $l_border, $r_border, $fill_size) = (1, 1, 1, $tot, $n + 1);

    while ( $cnt <= $tot ) {

        ##### Moving right #####

        # Step 1, decrease fill size
        --$fill_size;

        # Step 2, fill in
        for ( 1 .. $fill_size ) {
            $spiral[ $index++ ] = $cnt++;    
        }
        --$index;

        # Step 3, skip by n's
        for ( 1 .. $fill_size - 1 ) {
            $index += $n;
            $spiral[$index] = $cnt++;
        }

        ##### Moving left #####

        # Step 1, decrease fill size
        --$fill_size;

        # Step 2, fill in
        for ( 1 .. $fill_size ) {
            $spiral[ --$index ] = $cnt++;
        }

        # Step 4, skip by n's
        for ( 1 .. $fill_size - 1 ) {
            $index -= $n;
            $spiral[$index] = $cnt++;
        }
        ++$index;
    }
    shift @spiral;  # Arrays are 0 based
    return @spiral;
}