Given the OP's examplei input

With the grep

$message = "\n +_ ABC1_\n2 3 4";
print join'|', ( grep $_, split /[^A-Za-z0-9]+/ , $message )[0,1];

ABC1|2
[download]

Without

$message = "\n +_ ABC1_\n2 3 4";
print join'|', split /[^A-Za-z0-9]+/ , $message;

|ABC1|2|3|4
[download]

You'll notice the null leading element.

The list slice is pretty redundant, but it does make it obvious that you are only wanting the first two.