I would like to make a few comments about map. All maps can be written as "foreach" loops. Use "for" instead of "foreach" if you want - these keywords mean exactly the same.

The code that you started with uses what is called a "bare map", a map which does not make use of the potential output list. Most maps are like: @out_array = map{func..}@in_array; In your case, there is no @out_array and the map function works because of side-effects within the map's code. In this case $val is being modified.

I re-wrote the map code into a foreach loop as a demo. This allows you to add print statements to see what the loop is doing. And of course since this is a foreach loop, the $_ variable can be some other name (also see below). Performance between a map and a foreach version should be similar.

I personally only use map for short transformation operations, perhaps making a hash table from a list of values my %hash = map{$_ => 1}@list;. In this hash table case, the code is a "one liner" that makes use of the map's output, i.e. it is not a "bare" map. I don't use map's that do not make use of the left hand assignment. I have seen map functions with 20 lines of code. Again, I would recommend a foreach loop in that situation.

I hope the demo code below helps:

#!/usr/bin/perl
use strict;
use warnings;

my $octetstr = join '', map chr(), 0xab, 0xcd, 0x00, 0x1d, 0x94, 0x56;

my $val;
map { $val .= sprintf("%02x",$_) } unpack "CCCCCC", $octetstr;
my $mac=join(".",unpack("a4 a4 a4",$val));

print "MAC1: $mac\n";

########################
####  Exactly the same thing with foreach instead of map{}
########################

$val='';
foreach my $decimal_value (unpack "CCCCCC", $octetstr)
{ 
   $val .= sprintf("%02x",$decimal_value); # makes 2 digit Hex

   print "Decimal_value: $decimal_value \tval(hex string)=$val\n";
}
my $mac2=join(".",unpack("a4 a4 a4",$val));
print "MAC2: $mac2\n";

# the a4 unpack for $mac2 could be done in other, slower ways
# like with a regex (a very,very slow way, but just a hypothetical)
#

my $mac3 = $val;
$mac3 =~ s/^([0-9a-f]{4})([0-9a-f]{4})([0-9a-f]{4})/$1\.$2\.$3/;
print "MAC3: $mac3\n";

#######################
### BrowserUk, AnomalousMonk ideas with either H2 or H4
### far better!! One step, fast. Clearly the right way
### for this specific problem!
#######################

my $mac4 = join '.', unpack '(H4)*', $octetstr;
print "MAC4: $mac4\n";

__END__
All of these MAC's are the same:

MAC1: abcd.001d.9456
Decimal_value: 171     val(hex string)=ab
Decimal_value: 205     val(hex string)=abcd
Decimal_value: 0     val(hex string)=abcd00
Decimal_value: 29     val(hex string)=abcd001d
Decimal_value: 148     val(hex string)=abcd001d94
Decimal_value: 86     val(hex string)=abcd001d9456
MAC2: abcd.001d.9456
MAC3: abcd.001d.9456
MAC4: abcd.001d.9456
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