shmem has answered your specific question, but you could also consider some alternatives.

Several monks have recommended Perl6::Form as a viable replacement for format.

Whenever I think of displaying tables, I reach for Text::Table:

use strict;
use warnings;
use Text::Table;

my @s;
$s[0] = "123456789";
$s[1] = "987654321";
$s[2] = "123456789";
$s[3] = "987654321";
$s[4] = "123456789";
$s[5] = "987654321";
$s[6] = "123456789";
$s[7] = "987654321";
$s[8] = "123456789";

my $tb = Text::Table->new(
    \'| ', '', \'| ', '', \'| ', '', \' |'
);
for (@s) {
    my @grps = $_ =~ /(\d{3})/g;
    map { s/(.)/$1 /g } @grps;  # add space after each digit
    $tb->add(@grps);
}
my $rule = $tb->rule(qw/- +/);
my @rows = $tb->body();

print $rule;
my $r = 1;
for (@rows) {
    print $_;
    print $rule unless $r % 3;
    $r++;
}

__END__

+-------+-------+--------+
| 1 2 3 | 4 5 6 | 7 8 9  |
| 9 8 7 | 6 5 4 | 3 2 1  |
| 1 2 3 | 4 5 6 | 7 8 9  |
+-------+-------+--------+
| 9 8 7 | 6 5 4 | 3 2 1  |
| 1 2 3 | 4 5 6 | 7 8 9  |
| 9 8 7 | 6 5 4 | 3 2 1  |
+-------+-------+--------+
| 1 2 3 | 4 5 6 | 7 8 9  |
| 9 8 7 | 6 5 4 | 3 2 1  |
| 1 2 3 | 4 5 6 | 7 8 9  |
+-------+-------+--------+
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.