You want to add forty-two to every number in @numbers. So you use a map, naturally. With $_, it looks like this: map { $_ + 42 }, @numbers; with *, it looks like this: map * + 42, @numbers.

From the first example, we see that * is short not only for $_, but also for the surrounding {} block.

Now, think about what @numbers[$_ - 5] means. It means “the element in @numbers five places before what’s in $_“. (For the example, it doesn’t much matter what $_ itself contains.)

Conversely, what does @numbers[* - 5] mean? It means “the elemens in @numbers five places before *mumble-mumble*”. Or, in code, @numbers[ { $_ - 5 } ]. The mumbling comes from the fact that the $_ won’t be evaluated until the block is run, and a $_ value is sent in as a parameter. Now, it so happens that array indexings evaluate blocks by sending in their number of elements (their “length”) as a parameter to the block, so the code actually comes to mean @numbers[ @numbers - 5 ], or “the fifth last element in @numbers“.

Summing up, * means not just $_, but $_ in a code block which is possibly run at a later time, when it’s more convenient, with a value of $_ very much decided by the surrounding context.

Is “*” just a 50% shorter version of $_ under special cases? I can’t believe that’s all it is :)

Consider yourself having had a direct influence on a Perl 6 keyword!

for ($punch_card) {
  $_->bend;
  $_->fold;
  $_->mutilate;
}

I must admit that given is much clearer though, especially for those who don’t know the once-only loop idiom. I commonly use it when I need to perform several substitutions or transliterations on one string.

for ($phone) {
  tr{ABC} {2};
  tr{DEF} {3};
  tr{GHI} {4};
  tr{JKL} {5};
  tr{MNO} {6};
  tr{PQRS}{7};
  tr{TUV} {8};
  tr{WXYZ}{9};
}

However, for this example we won’t even need given in Perl 6.

$phone.=trans(
  ABC  => 2,
  DEF  => 3,
  GHI  => 4,
  JKL  => 5,
  MNO  => 6,
  PQRS => 7,
  TUV  => 8,
  WXYZ => 9,
);

I know that when is four characters, but it’s very easy to type on most keyboards because first and third characters are typed with the left hand, and second and fourth with the right hand :-)