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

$_ = '00a0c801adc6';

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

print "$_\n<BR>";

##</code><code>##

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