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use strict;
use warnings;
...

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

use strict;
use warnings;
...

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

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

my $number = 123;

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