0:13:46:b:4:31    # input
00:13:46:0b:04:31 # expected output

##</code><code>##

$mac =~ s/(^|:)([\da-f])(:|$)/${1}0${2}${3}/g;

##</code><code>##

0:13:46:b:4:31    # input
00:13:46:0b:4:31  # produced
00:13:46:0b:04:31 # expected

##</code><code>##

my @parts = split /:/ , $mac;
foreach (@parts) { if (length == 1) { $_ = "0" . $_; } }
$mac = join ":" , @parts;