As well as everything that's been said already, you don't need to use $_ explicitly very often. Usually when you see $_ actually mentioned, you can remove it. You don't need it, for instance in a pattern match or substitution, or as the sole argument to print:

foreach (<LOG>) {
  if ($_ =~/CRITICAL/) {
  print $_;
}
[download]

can just as easily be:

foreach (<LOG>) {
  if (/CRITICAL/) {
    print;
  }
}
[download]

Beginning perl programmers tend to like to leave $_ in explicitly, so they can see what they're doing, but it's generally better to leave it out and learn to read it implicitly yourself.

Tony