Because this method is destructive, I copy the @mac_add array before processing.

my @mac_add = qw/28 8a 1c 59 cc 85/;
my (@list,@parts);
@list = @mac_add;
while(@list) {
  push @parts, join '', splice @list,0,2
}
my $mac_R5 = join '.',@parts;
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