try

#!/usr/local/bin/perl -w
use strict;

$_ = '00a0c801adc6';

s/([\w]{2})(?!$)/$1:/gio, "\n<BR>";

print "$_\n<BR>";
[download]

to explain the regex,

s/
    ([\w]{2})      # matches a pair of letters/numbers
    (?!$)          # matches NOT end of string (zero-width negative lo
+okahead assertion)
/$1:/giox;         # replace pair with trailing colon
[download]

all this assumes you don't have to untaint the mac address. otherwise, you'll want to do that before the s///.

~Particle