$mac =~ s/(^|\.)([0-9a-f]?)(?=[0-9a-f])/$2 || 0/eg;
That solves the original problem. ybiC pointed out that another format mac addresses come in has groups of 4 characters, separated by :. The amended question was whether both formats can be handled together. The following does that:
$mac =~ s/[^0-9a-f]*([0-9a-f]?)([0-9a-f])/($1 || 0) . $2/eg;
In both cases the delimiter is gobbled by not being cached in a backref, and if it does not appear in even groups then a 0 will appear.
Danger alert. In the pair "05" the 0 gets replaced, so if you wanted to use this to insert anything other than 0 you would need to not just check the truth of $1, but rather it's length.
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