Nice problem! and some nice solutions ... everyone loves a diversion ...

Here's mine. For variety, and cos I'm looking for a new job (wink) I went for an OO approach. Certainly not the least characters, and I'm not sure it adds much in readability, but something different. Perhaps not even that, as I suspect that it is really a dressed up version of the iterative solution that was given already. Anyhow, here it is. I had fun doing it.

The object itself:

package SpiralSq;

use strict;

sub new
{
    my ($class, $size, $offset) = @_;
    $offset = 1 unless defined ($offset);

    my $self = { 'size'=>$size,
         'offset' => $offset,
         'nextoffset' => (4*($size - 1) + $offset)};
    bless $self, $class;
    return $self;
}

sub getInside
{
    my ($self) = @_;
    $self->{'inside'} = new SpiralSq($self->{'size'} - 2,
                     $self->{'nextoffset'}) 
    unless (defined $self->{'inside'});
    return $self->{'inside'};
}

sub getRow
{
    my ($self, $r) = @_;
    my $o = $self->{'offset'};
    my $s = $self->{'size'};
    my $n = $self->{'nextoffset'};
    my @row;

    if ($r == 0)
    {
    @row = ($o .. ($s - 1 + $o));
    }
    elsif ($r == $s - 1)
    {
    @row = ((2*($s - 1) + $o) .. (3*($s - 1) + $o));
    @row = reverse @row;
    }
    elsif ($r < $s)
    {
    @row = (($n - $r), 
        $self->getInside->getRow($r - 1), 
        ($s -1 + $r + $o));
    }
    else
    {
    die "error!";
    }
    return @row;
}

1;
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And a test script:

#!/opt/perl/bin/perl -w

use strict;
use SpiralSq;   

my $n = 18;

my $s = new SpiralSq($n);

for (0 .. ($n-1))
{
    print join " ", $s->getRow($_);
    print "\n";
}
[download]