use strict;
use warnings;

my $number = 123;
my $pad = 9 - length $number;

$number = '0' x $pad . $number;
print "$number\n";

##</code><code>##

use strict;
use warnings;

my $number = 123;

$number = sprintf "%09u", $number;
print "$number\n";

##</code><code>##

$number = 123;
$number =~ s/(\d{0,9})$/'0' x (9 - length $1) . $1/e;
print "$number\n";

##</code><code>##

my $number = 123;

print "$number\n" if ($number) = ( '0' x 9 . $number ) =~ m/(\d{9})$/;