What BrowserUk said. And if for some reason you really want the address in two-byte groups:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $octetstr = qq{\xab\xcd\x00\x1d\x94\x56};
 ;;
 my $mac = join '.', unpack '(H4)*', $octetstr;
 print qq{'$mac'};
"
'abcd.001d.9456'
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Update:

Why do i get an error when I start my code like "map { my $val .=" instead of declaring $val separately.

Because if your code were
    map { my $val .= sprintf("%02x",$_) } unpack "CCCCCC", $octetstr;
    $mac=join(".",unpack("a4 a4 a4",$val));
then the  $val lexical scoped within the map block is entirely local to the map block and does not exist outside of it, so the  $val variable in the
    $mac=join(...,unpack(...,$val));
statement is undeclared and is a syntactic error under strict (if that's the error you're getting) — unless you have an unrelated  $val declared somewhere, which would be a semantic error.