my @dates = qw| 10/13/2006 1/13/2006 10/1/2006 1/1/2006 |;
foreach my $date (@dates) {
    if ($date =~ m|^(\d{1,2}) / (\d{1,2}) / (\d{4})$|x) {   ## <-- anchoring
        $date = sprintf "%02d/%02d/%04d", $1, $2, $3;
    }
    print "result: <$date>\n";
}
__END__
result: <10/13/2006>
result: <01/13/2006>
result: <10/01/2006>
result: <01/01/2006>