Another way using the transliteration operator. Assumes a 12 digit address.

my @outputlist = '28:8a:1c:59:cc:85';

for my $addr (@outputlist) {
    my $mac_R5 = join ".", $addr =~ tr/://dr =~ /..../g;
    print $mac_R5, "\n";
}
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Update: Changed the transliteration to bind with the regex match. The former line was:

my $mac_R5 = join ".", $addr =~ tr/://d && $addr =~ /..../g;

Which works for perls before version 5.014.