my $re_mac = '(?: ( [0-9A-Z]{1,2} ) : )' x 5 . '( [0-9A-Z]{1,2} )';
$re_mac = qr[$re_mac]ix;
....

$mac =~ s[^ $re_mac $] [ sprintf '%02s' x 6, $1, $2, $3, $4, $5, $6 ]ex;