I will (too) try to explain and detail almut's answer:

my @mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03C");

sub convert {
    return map { "AP" . join ".", unpack "(A4)*", lc $_ } @_;
}

print "$_\n" for convert(@mac_addrs);
[download]

Well:

•  @answer = map { do_something_with("$_") } @array transforms one @array into another array @answer where each element of @answer is the result of the transform do_something_with(). Try this:

  $ perl -le '@a = (2, 3, 4, 5); @b = map { 2 * $_ } @a; print for @b'
  4
  6
  8
  10
  [download]

  Notice that @b has each of the elements of @a, doubled. The result of the block given to map can be an array, too, so:

  $ perl -le '@a = (2, 3, 4, 5); @b = map { $_ % 2 ? () : ($_, $_ / 2) }
  + @a; print for @b'
  2
  1
  4
  2
  [download]

  Notice that it printed the number and its half -- for the even numbers in @a.
• So, let's try the knowledge of map:

  $ perl -le '@mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03
  +C"); @a = map { lc $_ } @mac_addrs; print for @a'
  0015faa3f03a
  0015faa3f03b
  0015faa3f03c
  [download]

• Now, let's add unpack"(A4)*" to the equation (unpack any number of groups of four alphanumeric chars):

  $ perl -le '@mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03
  +C"); @a = map { unpack "(A4)*", lc $_ } @mac_addrs; print for @a'
  0015
  faa3
  f03a
  0015
  faa3
  f03b
  0015
  faa3
  f03c
  [download]

• Now, let's join the parts with ".":

  $ perl -le '@mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03
  +C"); @a = map { join ".", unpack "(A4)*", lc $_ } @mac_addrs; print f
  +or @a'
  0015.faa3.f03a
  0015.faa3.f03b
  0015.faa3.f03c
  [download]

• And then, add the "AP" in the beginning:

  @mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03C"); @a = ma
  +p { "AP" . join ".", unpack "(A4)*", lc $_ } @mac_addrs; print for @a
  +'
  AP0015.faa3.f03a
  AP0015.faa3.f03b
  AP0015.faa3.f03c
  [download]

• voilą!! you already have your answer. Now, package everything in a neat subroutine:

  $ perl -le '
  > use strict;
  > my @mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03C");
  > sub convert {
  >   map { "AP" . join ".", unpack "(A4)*", lc $_ } @_
  > }
  > print for convert @mac_addrs
  > '
  AP0015.faa3.f03a
  AP0015.faa3.f03b
  AP0015.faa3.f03c
  [download]

Got it?